Coelocanth: Can't recall the class levelling tables off the top of my head, so this may not come into play in BG due to the level cap, but at some point, you only get a set number of HPs per level and no longer get a die roll.
It happens around level 10 for all classes; only thieves, bards, and druids can hit 10 in vanilla ToSC. You'll hit it almost immediately in BG2, though. Warriors will get 3 hp per level, Mages and Sorcerers will get 1 hp per level, and Priests and Rogues will get 2 hp per level.
In short, if you're planning on importing a character to BG2 in the future, make sure s/he gets good hp rolls in BG!