I believe what OP is asking is if damage is divided among enemies instead of doing full damage to each one. Kinda like certain moves in Pokemon that hit the entire field; the damage is split amongst all the Pokemon that take damage. The fewer Pokemon currently in battle, the more damage that's taken by those still in battle.
I'm pretty sure each enemy takes full damage from every attack. Some things will scale based on how many enemies they hit, which means they do more damage the more enemies there are taking damage. But as far as I know, you don't effectively "lose" damage for hitting more targets at a time. If this was the case I think DPSing in dungeons with large numbers of trash mobs (like Fungal Grotto's crab boss) would be much, much less effective.