According to Sloot, Delves enemies actually have less health and deal less damage damage with 2 players, then doing it solo.
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.
Doing some Delves with 4 and 5 players, we came to the following multipliers for these number of players as well.
It definitely seems unintended when Delves were marketed as solo content, but there is one notable exception, Crazed Abomination in the Underkeep. These for some reason have an incredible amount of health with a 5 man stack having almost 10x as much health as a solo. While it does take longer in a group, its still much easier than doing it solo for most classes.