It seems to be a global thing.
So if Mob/player X has 5021 HP, his bar is X distance long. If he gets essence it gets longer.
However, the difference is SO MINUTE that it's almost unnoticable. (Buffing myself on test with Behemoth, Mongo Crush, Coruscating screen, makes only a slight difference to the length of my bar, even though it's 3800 HP or so)