Double the weight of a bullet and you double it's energy.
Double the velocity of a bullet and you quadruple the energy.
This is true, but doesn't tell the whole story about energy, or how it is weighted in favor of velocity. The formula for kinetic energy is:
1/2 mv2
Or one-half of the mass times the velocity squared
This means, triple the velocity, and you get 9 times the energy
Triple the mass, you get one and a half times the energy.....not so good
As far as bullet effectiveness, it doesn't tell the whole story by far.....but it is the quickest metric to look at. Obviously, we need a better metric....but so far, we don't really have one, as far as I know