I suspect that the .308 the guy used technically had more energy than what appears to have been a trapdoor .45-70 load. So why doesn't it penetrate as well? Becasue the lighter, skinnier bullet tumbles and breaks up, while the heavier bullet stays together?
I remember reading an article by Chuck Hawkes where he basically dismisses momentum (velocity x mass) as a measure of bullet effectiveness, in favor of energy (velocity squared x mass). But at least in terms of penetration, momentum seems to be a better predictor. Momentum is a measure of an object's inertia which is basically an indicator of how much force is needed to change its state of motion (velocity/direction). The more momentum a projectile has, the harder it is to stop (the freight train versus race car analogy comes to mind).