What if you said that a light weapon has a -2 to parry a two handed weapon, and a two handed weapon also had a -2 to parry a light weapon? I used to play an RPG with a similar rule (Quest), and it seemed to make sense.
Quest also had a sweet table where you compared your attack rating to your opponent's dodge or parry (sound familiar?), rolled a d100, and that told you not only if you hit, but where you hit (head, weapon arm, right leg, etc.). You'd get a greater percent of head shots as your skill went up, for instance. If you opponent was more skilled than you, you'd hit less and most of your hits would be to the legs and arms.