The wound volume on a live target is essentially a by-product of bullet diameter, bullet weight, and velocity. Of these 3 elements bullet diameter, or rather a bullet's frontal area, is by far the most important. If this was not the case there would be less effort in designing bullets that expand reliably. The majority of the very best big game bullets available produce about 1.5X expansion, and retain a large amount of their original weight, some 85-90%, which produces sufficient momentum to ensure deep, straight line penetration, but more on that later.
If we compare the frontal area of .308" and .458" bullets we get .024 square inches for the .308 and .052 square inches for the .458. That means that the .458 will displace a little more than twice the tissue and bone as it moves through the target, than will a .308, all else being equal. But what about velocity, a .308 bullet might be twice as fast as one fired from a .45/70! This is true, but it will also be lighter, so the momentum is created by velocity rather than mass. It is possible that the .308 bullet and the .45/70 bullet will penetrate equally in a game animal, it is equally possible that both bullets will exit, particularly on a broadside shot, provided both bullets perform as designed. Provided a bullet is supersonic when it hits it's target, a shock-wave crushes and displaces soft tissue and again the bullet with the greater frontal area produces the largest displacement of tissue, and the advantage of 1000 fps for a small bore doesn't make up for the frontal area of the large bore, although an impact velocity of about 2000 fps seems to maximize the potential of the shock-wave. You might point out that varmints are virtually blown to pieces by high velocity impacts by light weight bullets from .17s and .22s. But those bullets are designed to virtually explode on contact with targets of very light resistance. Such a bullet would produce a nasty but superficial wound on a big game animal. When bone is hit by a properly designed big game bullet, the shock-wave has no effect other than to scatter bone fragments in it's wake. A shock-wave will not break bone, but a bone hit by a larger bullet produces a more complex wound, as a larger volume of bone is scattered through the tissue, resulting in greater bleeding.
Straight line penetration is important to ensure that the bullet damages the vital organs we intend it to. If your bullet enters the target and deflects following a path of least resistance, it may not damage the heart or lungs, the result being a wounded animal who will take longer to succumb to its wound. Bullets kill by interrupting the flow of blood enriched with oxygen to the brain, without oxygen the brain dies, and this might happen quickly or slowly depending on the damage our bullet does. To that end, expansion produces several advantages: it makes a larger wound due to its increased size, the center of gravity of the bullet moves forward making the bullet more stable, and the bullet becomes shorter which again aids in stability provided it retains a linear axis to rotate around. If the bullet shortens too much and lo linear shank is left, stability is lost.
Goes about a foot and a half into a dead fir tree. 
