There are a lot of factors that go into the formation of the EXP gained, some are:
-Your level
-The monster's level
-Who made the final blow
-If you used a GF (sometimes)

I remember once I was fighting a Malboro on a low level file, and I could finally start leveling up Quistis. So I did, I Lv Uped the Malboro to like level 60ish, then killed it. Quistis received 8000ish EXP for that fight. Enough to level her up quite a bit.