When we go out into the field carrying a .460 Rowland for protection against bear attack WHAT ammo do we prefer? A 255 grain hard cast lead round nose, flat point. This bullet is NOT designed to expand, but instead designed to punch THROUGH to the vitals and beyond.
The fact is, humans are basically the same size as a small bear, a mountain lion, a rogue deer...etc. Thus, when using "lethal force" against a human why do we "presume" a 9mm is "enough?" We would certainly NOT presume that for a 200 lb white-tail deer!
Even IF we compare non-expanding bullets, "magnum" level handgun bullets tend to be far more effective than sub-magnum calibers. The "problem" is that calibers sufficient to reliably put down a large human don't fit within the "self-defense" carry paradigm.