A lighter bullet does not neccessarily mean higher velocities. The velocity of the bullet is derived from the pressure created in the barrel. Higher pressure tend to produce higher velocities.A heavier bullet is harder to push out of a barrel then a lighter one is, therefore creating more pressure as the powder is ignite. The more preesure therefore can creat higher velocities in a heavier bullet in the same caliber. I know it does not all add up, but there is some truth to it.