The Legendary Pixel Crew

Well, even if you have to split the rewards in half, a double sized party could get more than double rewards with less losses. It should also be noted that, no matter how many ways you have to split a reward, it will always be more than failing to get anything at all. (And no, splitting something infinite ways isn't even a possibility to have to consider, though if you have a high level mathamancer it might work out somehow)

If you have infinite participants, and the GM is generous enough to add a single copper piece to the loot for every participant, all participants that survive can have a million gold pieces, as for every participant you can pick a million gold pieces worth of copper out of the loot pile and not run out of things in either pool. (Just like how there is an equal amount of even integers as there are integers in total.)

As for the current scenario, I think what's most important is that this dungeon is a meat grinder for pcs. People are gonna die. If your chances of survival are a little higher than the other party's, but not so much higher that they're likely to die quickly and leave you alone with extra tough monsters, you'd be effectively getting a share of their loot as well as your own.

A downside to the infinite party scenario is that all loot takes up some amount of space, and even if the dungeon is filled with the most value you can fit into every square inch, as people continue to take their loot you will still eventually reach a point where it takes longer to reach the loot than it takes to reach the end of someone's natural lifespan. A finite number of people will be able to take their loot within their lifetime, but an infinite amount of people will not. This means that effectively 0% (a finite number divided by infinity) of all people will reach their loot within their lifetime. Therefore you should only go on an infinite-person quest if you have an unlimited lifespan(as well as an infinite/replenishable food source), and nothing going on with a time limit.

Ah, but if you have greater teleport, you can teleport any distance. With an infinite dungeon and infinite participants, there are an infinite number of people who can cast that spell and get to the loot.