Yep, What mookanana said.
A chaos spawn or rat ogre seems more threatening on its own, depending on the area, but generally if you run away holding blovck theres no way for them to damage you, so one player can easily hold aggro while the others deal with the horde and specials. A troll on the other hand has some special tricks like the puke, and that brutal overhead smash, so you need to always be aware what its doing.
Additionally, the double up on healthbars really increases the amount of time needed to kill it relative to other bosses, in spite of the smaller HP pool. 4 players whaling on it gets past the breakpoint of it only occurring once during the fight, so its not as prevalent, but as soon as people get distracted by other things the time-to-kill balloons way out, leading to more hordes and specials spawning etc.
That said, I would agree with increasing its HP by 10-15% or so.