The bullets we have available today are far and away better than what was available when John Browning designed the 1911. But, a 9mm has to expand by 27% to be the size a .45 starts out. Given that same 27% expansion, the .45 is .573".
I don't know if there's a real difference when shooting 2-legged predators or not, and I hope never to find out, but I have some experience shooting wild animals with revolvers. When speaking in terms of handgun "killing power" there is no question in my mind that bigger is better. Said another way, big bullets let in a lot of air. I guess what that means is that the first consideration regarding gun fighting should be: Bring a gun. After that, it should be the largest caliber you can EFFECTIVELY use. If all you can do is make a lot of noise that's probably not effective. Everything involved with self-defense weapons is a compromise of some sort, and it is very important to be proficient with what you carry. Jeff Cooper said a handgun is what you use until you can get to your rifle. I think he may have been onto something.