Mr. Meyer,
There were two studies after Korea, one by the U.S. Army and one by the Israelies. Both concluded that in combat, hits were made by "accident", not aimed fire. Thus, smaller rifles that fire more projectiles create more "accidents", leading to the M16 family.
"Morbidly Obese" (or "Bariatric" in the EMS world) folks are a problem - a growning one (pun intended). There are several documented failures to stop with 'pre-fragmented' rounds where all the projectiles (shot) were stopped in the fat layer. Since we do not get to choose out attackers, this is becoming a more significant factor in defensive ammo selection.