Multi-player Scaling?

What is the deal woth the multi-player scaling. It seems like for having a single friend in the same area as you enemies get 10x the health and deal double damage.

I wanted to do an essence drain contagion build but with the scaling essence drain and contagion on a single non elite mob will barely kill it. And with the durations only being around 5 seconds the upkeep os a huge chore since I can't get resets on the ai.

Is the multi-player scaling bugged I get not wanting players to just be doing double damage but the boss fights take so long that it's just tedious especially woth the enemies hitting harder too.