Enemies gain +100% base health per player. So when you have a full team, enemies have 4x the amount of health they would in single player. I do not know if they gain increased damage, but I suspect they might. They do not gain increased health regen or anything of that nature, although in UVHM, since their regen is based on max health, in a sense they do. In addition, in UVHM all enemies will scale to the highest level player, making them MUCH more difficult if the team isn’t equally levelled. Not sure if this health formula applies equally to the Raid Bosses, but it is the way it works for standard mobs. In addition, in certain places during the story (rescuing Lilith, after destroying the beacons with Brick, etc) there are guaranteed baddass spawns which increase in number depending on the number of players. And of course Varkids have a higher evolve chance as well, and Tubbies spawn more frequently. With all of these changes, however, the game is still easier in co-op because of the division of enemy aggression. I hope this answers your question.