I think the basic concept here is that both calibers are usually restricted to a overall length of 2.800 by the magazine box of a short action. So when one bullet is longer and slimmer than the other, the longer bullet has to be seated deeper in the case.

Having said that, however, I believe the increased bearing surface of the longer bullet has more to do with creating pressure than seating the bullet deeper. I have built some rifles on long actions and chambered them for short action cartridges - just so they could be seated out to as long as the throat would allow. And also played around with custom long throating. The extra length does affect the pressure to velocity ratio, but its not a real big deal. Maybe 50 fps in a medium caliber.