How does enemy difficulty scale up relative to the number of players in co-op?

Question

If another player joins my game (through LAN), a message saying something like this appears: "Player 2 has joined your struggle. The creatures of Pandora grow stronger." If I'm the only one left in the game, a message appears, saying: "The creatures of Pandora weaken."

A loading screen also shows this:
"More players means more enemies."

How does the game increase the number of enemies per player?
How does enemy difficulty scale up relative to the number of players in co-op?

Answer 1

In my experience, when there are at least 3 players playing at the same time there can also be different enemy types that will spawn. For example in, The Highlands, near the row boat at the edge of the map a Badass Pire Thresher would spawn only if you had 3+ players in game.

Answer 2

the more people the bigger the health bar instead of having a bandit you might have an armored bandit more often. Not every player has more health you just get more badass's you also get more loot and xp though so its worth it.

Answer 3

I also was interested in this question, and having failed to find a real mathematical answered, here's what I understood:

enemy difficulty (and enemy numbers in some cases), depending on the patch, is given by, in order of importance:

• the zone you are in;
• number of players;
• average player level;
• average badass ranking (unsure, but likely).