The 308 is a smaller (Volume) case but has a higher pressure limit. The higher pressure limit can get a bullet accellerating quicker than a lower pressure cartridge.

The 06 has a larger volume case but a lower pressure limitand can maintain pressures longer (further) down the barrel.This willaccellerate the bullet for a longer duration. This means an 06 will not get to fully accelerate the bullet and the 308 will come closer to fully accelerating the bullet with the available powder. Powders in the 308 tend to be faster burning also.