I can't give you an exact answer because I am not a scientist, nor do I completely understand the physics of gas. But I can give you my understanding of it, which I'm sure someone will chime in and tell me I don't knwo WTF I'm talking about.
Of course theriare many variables at play...however, when priming a keg, you have a fairly small headspac which plays a role in the carbonation of your beer, since once the headspace is compressed with gas, it can then beginto be absorbed into solution, (albit at different rates dependent upon temperature). The volume of beer, the volum of headspace contributes to the difference in carbonation volumes between bottles and kegs.
I guess it partially comes down to volumes of liquid vs. air/co2. Again, many variables are at play, but a single larger voulme is much more "efficient" than many smaller volume sif that makes sense....**** it I have had a few beers tonight, and I don't think I can do you justice, but maybe the coments I have made can get yout to think of things in a slightly different manner.
Cheers! sorry I am druK