Lord of the Rings is one of the works which explain this: if you want to pass unnoticed, the lesser the better.

A large party requires adequate logistic, preparation and support. Not ideal if you want to stay below the radar.

A small team, or even a single person, can more easily operate.

If your monsters have the capability to detect large vehicles and groups, this will make smaller groups more viable.

Think of a Gold Rush.

Sane professionals will proceed slowly. Clear an area on the edge of the territory, exploit it, keep it cleared, rinse and repeat.

At the same time, the desperate and foolhardy go in deeper. Go in, dart around, grab something, run away. These people are not patient and organized enough to "play it safe." They want or need instant gratification, no need to share the riches they will surely find next time, for certain. They feel it in their veins, their luck will turn if they go there one more time. Why, Smelly Jim got himself a crystal to buy his own castle.

Read about Klondike, Nome, California.

The monsters are dangerous enough that numbers don't significantly increase the odds of a party's success.

If a fire elemental can kill a small army just as easily as it can kill 1 man, then the smart strategy is to split up into very small groups that you don't invest much backing into individually. If you send in 50 small groups knowing that only 40 will come back, you can form a business plan around using the survivors' profits to replace your losses, and go again. If you instead send in one really big group and it gets wiped, you lose your whole business.

Food and distance; any long expedition must eventually live off the land rather than supplies they bring with them. So if the area of devastation is wide, meaning that expeditions must travel far from the last base of supply, and edibles within the area relatively rare large caravans can't access the gem fields. They can't haul enough food to get there before they eat it all and neither can large parties, only small groups can live off the land and get in and out fast enough to succeed. This can also be done by restricting access to drinking water, which is often polluted by volcanic gases or chemicals leeched from a volcanic ash layer.

To a large degree this will be a temporary situation if the fields appear to be a longterm profitable venture. People will build infrastructure to exploit the site if it's worth it.

For your trivial example, geography would be an important enough factor on it's own. Maybe those mountain routes are too narrow in some places for large groups to keep fast pace. The people up ahead need to wait for the people down behind to catch up. A longer column would take more time to cross the same ground than a shorter column.

This goes for any other scenarios where there would be such "bottlenecks". For instance, a shallow river boat can only carry so many people at any one time. A cave with a few narrow passages can only fit so many people before they started getting in each others' ways. A frozen lake can only support the weight of so many people at the same time.

What if the dangers are not only monsters that can be fought, but plain weird? Think of cobwebs that kill anyone who touches them ... three weeks later. Think of an invisble vortex that pulls anyone who enter into the air, and kills them, but needs 20 minutes or so to "recharge". This means only those with intimate knowledge of the territory can navigate it, and they will be careful in with whom they share this knowledge.

Or some artifacts are rumored to be incredible powerful. Who would risk sending anyone but the most loyal follower on an expedition, when the expedition leader might gain the power to have one wish granted?

Or those who explore the terrain carry home a strange curse: Their children might be born with strange afflictions and suffer from mobbing throughout their lives. The ghost of their mother in law moves in with them. Those who spend significant time in this area will experience a haunting (or addiction) to always return. Quickly, this will be known or guessed at by everyone - so only those who are allready a little crazy will enter the territory, or let themselves be recruited.

None of the above ideas are mine, I lifted them all from the Strugatzky' brothers novel Wayside Picknick (which inspired the movie, and the games, Stalker).

If the terrain is inhospitable, a large force can be a major drawback

As the romans found out

If the people know the land, they can set all sorts of traps which will make your numbers count against you, making it impossible to split up, difficult to coordinate, and easily picked off by flinging thins into the crowd.

Perhaps human beings emit some sort of 'radiation', perhaps magical of origin that draws monsters towards them. One person or small groups, fewer than 10 or so might have limited problems as their energy signature is too difficult to make out from background radiation, but the moment really large groups try their hands at entering the lands they can expect to become monster magnets.

For example: Something like the grimm from RWBY. The grimm are attracted to negative emotions, the more negative emotions there are, the more grimm will come. More Grimm means more negative emotions (anger, fear,...) and the vicious cycle has begun. The main problem the people from Remnant have with Grimm is that once such a cycle starts, it's pretty difficult to break it on time.

You will need some sort of reason though for how monsters survive. The grimm are pretty simple. They exist out of darkness, so they don't need sleep, food or water. Only shelter, because in one of the World of Remnant videos, the one about Atlas we see Grimm freezing to death. Your elementals will have no problem, but if you want an actual living and breathing T-rex or something, you will need to explain how a T-rex gets sufficient food. Of course all these creatures could just feed off of the magical radiation of the land or something like that.

I hope this helps.

Maybe the investors are cheap

You might think someone rich who is sponsoring an expedition would fund it generously enough to have be able to take care of itself, but you might be wrong. Someone who doesn't really know what's involved might decide on the size and composition of an expedition on the basis of their own mistaken beliefs. And they're more likely to actually send the expedition if they believe it can be done cheaply.

Or, think of really big anteaters

A predator which eats prey much smaller than it is will eat large numbers of them. It might not bother to feed on a small party, but a line of a hundred little morsels would be worth stopping for.

This has the additional feature that large parties can fragment. A large party starts out, and something big shows up and starts eating people. What do the as-yet-uneaten people to? They scatter. And if they survive until morning, you have a number of small parties, all poorly equipped and with haphazard composition.

Laws or Diplomacy

Laws could restrict major corporations and state forces from entering the area. Maybe the surrounding states fear magical contamination from artifacts of the 'zone' or hold an anti-individualist credo that make them wary of the social disruption caused by returning 'nouveau rich' adventurers.

Alternatively, if more than one state borders on the territory they could come to an agreement not to exploit it. Perhaps the sudden potential of the 'zone' brought them to the brink of war and they agreed to stay out in order not to spark conflict.

In either scenario the territory would be avoided by state forces and 'respectable' businessmen who needed to work within the law. Instead exploration would be spearheaded by marginalised down and outs or by shady business concerns unconcerned with legality. Depending how seriously the laws/treaties were maintained you might also see plausibly deniable shadow actors such as privateers and government-backed mercenaries taking a cut for the governments that publicly banned exploration.

Logistics, mobility, discretion, sanitation, scarcity, Preservation of life

Logistics

The bigger the group, the more food and equipment they will require. All of which will need to be carried. Even if you can scavenge food and potable water en route, the bigger a group you need to feed the harder this task becomes.

Mobility

Again, a smaller group moves faster. You need less time to set up/drop camp for the night, and don't need to wait around as long for the rest of the party to cross obstacles.

Discretion

If the area to be explored is full of dangers, you are more likely to be able to avoid them in a squad of 5 than 50. Especially if these dangers include other, hostile scouting parties. You may be less likely to survive such an encounter, but that's the balance.

Sanitation

A small group can dig their own holes to poop in, or do as they please. Once your group gets bigger, you need to worry more about designated spaces or risk the spread of disease.
(Historically, diseases like Dysentery were huge problems for armies on the march)

Scarcity

These resources, while valuable and accessible, aren't found that densely. It's simply not worth funding a large expedition to scour an area when a few individuals can get cover the same ground faster and achieve the same aims

Preservation of life

You don't know what's out there, but you know fur sure it could kill you in seconds. Losing 5 people from your population after they are lost on expedition is not as big a blow as losing 50.

Or maybe you could orientate yourself on the anime/manga made in Abyss,
where there is the abyss where only a few people are able to withstand the so called curse of the abyss.
ypu can make it layered, like in the outskirts it's less dangerous and more people can go, but the farther from the middle the less precsious minerals, artifacts what ever.
but closer the mor valuable the more dangerous. and many people die, get injured just because they moved closer to the curse. Or Big groups could unwkwon worsen the curse.

Spoiler:

The curse is a force field wiche gets stronger the close to the core, and the stronge the more severe the burden on body and soul.

I want to do a parallel on Roman history.

After conquest of Gaul, Roman secured eastern front with german tribes and bedded on Rhine river. why don't fight german tribes on east ? I mean with a full conquest?
Because there was nothing. No big cities, no roads, no infrastructure, nothing to conquest.

So I have a question for you: who said these "new" territories are full of monsters, treasures ? Someone has to go ther and come back, right ?

Let's say someone go ther and come back with stories of treasures and wonders.
Now what? Many dreamed these treasures, but it's hard and is not for everyone!
You have to be:

• prepared to survive in a land with no roads,


• no clues on geography: no maps. Where you find water? A shelter?


• no common language when you find humans,


• trained for combat (you mentioned monsters, right ? I will add lone people, maybe little bit crazy one? )


• trained for hunting


• resist to new diseases


• food for your horses (and water! and a shelter)


• motivated

In conclusion: I want to emphatize on motivation side. After one, two, three ( maybe a month) of exploring a land full of monsters, with no much food or water, no fun ( remember? night could be very long if you are in an unknow place every night..) is for sure stressful.. so small groups, highly motivated is right solution, for exploring

The simple fact is larger groups have a larger more detectable foot print than a smaller group.

First, just the noise levels and the vibrations on the ground that 50 people cause versus 1.

Then there the pollution, waste products, and smells. A large group disrupts a large portion of the forest as it goes through and is far more noticeable.

Cooking fire for 1 vs 50 is going to be way more massive, and the scent and smoke will travel everywhere. A single adventure might limit themselves to precooked food they can eat without starting a fire.

Everything is larger, bathrooms for 50 is going to generate tons of waste and probably emit a large amount of smell. Way easier for an animal(s) to track.

They will form a large camp which will be super easy to spot from a distance.

Our own body oils will ruby over everything we touch and brush up against,break branches, and etc, forming an easy path for animals to follow.

So your creatures are variable sensitive to sound,smell,and vibration depending on the group size you want to be attacked. Also it doesn't hurt if the animals are larger so they have a bigger appetite thus there hunger will be better satisfied by eating a group of 50 adventures vs 1 which isn't even a snack.

The biggest problem here is that more adventures will be better equipped to handle larger more vicious enemies. A party of 50 can have 20 machine gun operators, and 5 missile launchers.

I am thinking like in the movie "tremors" with the large hazardous earth worms that are sensitive to just the things we are looking for here. While its underground its pretty much safe from machine gun and missile attack. Otherwise we would just drop bombs on them from 2000 feet up where they can't reach us. Even if some of your animals can fly they can't keep up to a plane at 500+ mph.