Kinetic energy is a poor measure of effectiveness. Compare KE= 1/2m*v^2 to momentum P=mv. Significantly different weighting of the variables, but regardless inertia and momentum are going to be extremely important because the impulse of energy is important, how long the energy is delivered over. A small high velocity round may do a lot of damage on impact but compared to a heavier slower round the delivery of KE is going to drop much more quickly. If you hit a large bone maybe, for example, the small fast round may do a lot of damage to the bone, but might fail to penetrate it and cause further damage. The heavier round might damage the bone less, but is more likely to continue through and continue its trail of damage.