It takes very little tolerance gap to get gas blowback. Why brass w/ enlarged pockets should be tossed. Your theory about smaller flash holes is interesting, but I am betting improper primer fit + high pressures.
"Given adequate penetration, a larger diameter bullet will have an edge in wounding effectiveness. It will damage a blood vessel the smaller projectile barely misses. The larger permanent cavity may lead to faster blood loss. Although such an edge clearly exists, its significance cannot be quantified".