There was a thread a while back here regarding the game's code, where a then-developer answered some of the questions. It would appear that npcs (all of them, even your allies) have 100% of their health on Padawan, 125% on Jedi and 150% on Jedi Knight. I have done some testing and this more or less checks out.
Bosses have 75% of their health on Padawan, 100% on Jedi and 125% on Jedi Knight. In my testing this wasn't true upon spawning known bosses in like Tavion and Alora, it might happen in their respective maps.
It is said that npcs that belong to force user classes don't have any health modifiers but upon testing that's incorrect - I was spawning them in on Jedi Knight difficulty and they have what seems like arbitrary health increases since I couldn't pinpoint an exact percentage. Cultist Saberists, for example, had around 115 health instead of their base 100; Shadowtroopers had 235 instead of 200 and Desann had 800 something (I don't remember the approximate value) instead of 500.
Anyone knows the real math behind this? Also I would like to know what sect of the code should I delete to remove this "feature".
EDIT: I found the developers' reply to my question, that's where I got the percentages you see here