Yes it has a bigeffect because kinetic energy essentially is massx speed x speed.But what has a huge effect is speed, so if you can lower the weight in something, throw it faster because it weighs less, you have much more kinetic energy.

Killing an animal needs enough weight to be effective. (to plow through bones, tissue, etc) Would you rather get hit in the face with a plastic hockey ball, or an NHL puck that is much heavier?

I'm not knocking KE at all, just saying that when killing animals it's not the end all be all. What really it all boils down to as the other poster mentioned, in my humble opinion, is what the individual archer can handle.