That I can believe. I'm no physics major obviously but I would have thought that the energy put out by an AEG would be the constant. If the energy output remains constant an object with less mass should be propelled with more velocity than one with higher mass. The reduction in velocity resulting from the increased amount of energy consumed to overcome the extra weight to start the object moving in the first place. The heavier object will have its own inertia which should be more than the lighter unit due to the additional mass and that is why it hits harder.
Oh hell where are Bill Nye the Science guy and the Mythbusters when you need them.