Generally that is true but there are some exceptions. I don't have the #'s on hand anymore, but some bows are so much more efficient with lighter arrows (near the 5 gr/lb range) that the speed increase they achieve generates more KE with the lighter arrows than heavier, when shot from the same bow. Norb Mullaney talked about this phenomena a while back in one of his articles in Bowhunting World, and I've seen the same thing in my own experience.