We released an article earlier today that explains multiple reasons why delving in a group is better. One of those is the enemy health and enemy damage scaling. It seems that Sloot has done some testing and determined that while 2 to 5 players seems to scale like you'd expect it to behave, the difference between solo and everything else is very odd.
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According to Sloot, Delves enemies actually have less health and deal less damage damage with 2 players, then doing it solo.