I've found the exp gained is directly proportionate to the max HP of the target in question, with modifiers based off what percentage of your max Hp their's is, weapons and level bonuses and null in this equation. Basically, if I attack someone with 800hp, I get ~275 exp, someone with 250hp, I get ~13exp, someone of same level as person with 250hp, but has 210hp, I get 1exp.
Make sense?