[Mr_Sandman]

A small coherent dungeon can have a group allied monsters that will gang up on the PCs, if they see them coming and get the word out to their pals, requiring more stealth and cunning to take out guards and small groups quickly and quietly, and avoid large concentrations. A mega dungeon, can have more physical separation between the monsters, and a variety of faction competing against each other in an uneasy balance, more or less like a city, or a somewhat geographically compressed above ground situation with rival ‘states’ and factions.

The stereotypical medium-sized dungeon, with a variety of unconnected monsters packed on top of each other needs a more unique explanation. One group might be in the middle of invading and taking the dungeon from another. In that case traps the adventurers fall into are there security against each other. Some of the non-sapient monsters might be kept as a food source (or some other resource) of a group of more intelligent monsters, or it might be a wild beast that they don’t want to lose members of their tribe fighting, but they have a secure place it can’t reach. In any case, I think this kind of situation is probably unstable, and in the game world, the PCs just happen to come along at the right time. If they retreat to the surface and come back later, things may have changed, and they could find themselves facing more unified opposition.