Monsters spawn more frequently, and in higher numbers in dark spaces inside a rather small world border (10-20 or so).

This is most likely because monsters, unable to spawn or dying quickly outside the limits, only manage to spawn and survive within the walls.

This causes many problems in surival/survival challenges where you can be hoarded by about 20 or 30 different monster inside a a small cave room, making clearing it nearly impossible without dying, changing to peaceful, or clearing with commands.