Take your baseball and throw it against your garage door than borrow your wife's bowling ball and throw that at the same door. Now tell me which one has more kinetic energy. The fast baseball with little weight or the slower bowling ball with 10 times the weight. Same same when throwing a javaline, shooting a bow or your favorite bullet.
If I was going to throw both of them at you as hard as I could, which one would you rather get hit with? The bowling ball because I only have X amount of energy with which to throw a blowling ball and while it my have more KE, I can't get enough speed from it to hurt you. With the baseball on the other hand I will be breaking some bones if I hit you with it. Those two comparison aren't really valid here considering the extreme differences in weight. All that being said you won't see more than 1-2 foot pounds of difference when going from say a 300 gr. arrow to a 500 grain arrow. Also, there is something called the law of deminishing returns. In other words a cam may more more efficient up to a certain weight and any extra arrow weight you add after that point will actually decrease efficiency.