I'm sure its more complicated than I understand, at this point in time, so ill only mention my opinion and thoughts on small rifle primers in a case the size of a 308.

The primer Mfgs do make a Sm Rifle primer in a magnum version, which may make up for its smaller size, and I dont know that a Lg rifle primer in a 308 is not more than it really needs to function well.

In cases like the BR cases,(same case head size as the 308) all use Sm rifle primers, and the flash hole is also smaller, this should propel the primers flash (fire) further into the case, than a large flash hole, with the same size primer.

And last but not least, using a Sm rifle primer in a case with a head diameter the size of a 308, or BR case for that mater, does leave more brass surrounding the primer pocket, and this may extend the life of the case, by not allowing the primer pocket to expand as quickly, become loose and shortening the case life.
Just my thought on the subject.
Mike.