At close ranges, the lighter pellet is probably going faster giving it more energy. At longer ranges, the pellets slow down considerable making the mass the more determining factor. Remember the energy is the mass times the square of the velocity. The mass doesn't change, so it always supplies its part of the energy equation. If you go out to the point where the velocity is 1/2 of the muzzle velocity, you have 1/2 times 1/2 of the velocity supplying its part of the energy, so you only have 1/4 of the energy you have at the muzzle. The farther out you go, the more the pellet slows down, and the more pronounced this effect is.