Sorry, but a 0.1 inch increase in diameter between a 9mm and a 45acp isn't really that big. Even with expansion of hollow points, the difference between a 9mm HP expanded and a 45acp HP expanded isn't all that much. The larger diameter bullets however are larger is mass by nature. I.e. the largest 9mm is basically a 147 grain, while the largest 45acp is basically a 230 grain. That is a significant difference mass. That means generally better penetration. It usually means velocity is maintained longer. And it means expansion is more likely. With smaller diameter and smaller mass bullets, you need more velocity to ensure penetration and expansion.
Of course, sometimes you can combine the two and get hellacious velocity and decent mass. Such as a 357 magnum. Same diameter as the 9mm; even the same weight bullet many times. But the increased velocity allows for more penetration and expansion.
But the fact still remains that unless you hit part of the CNS, the person shot is not going down like a "Sack of Spuds". I define this meaning "Immediately".
Everyone speaks of shot placement. And shot placement is undoubtedly the key to effective hand gun defense. However, your shot placement is still pretty much a ball park estimate. Even if you practice every day, that mainly means you have a greater chance of hitting your target, than the person who never practices and flinches or even closes their eyes. But even with the expert shooter, the person you are shooting is NOT going to just stand there and let you take that perfectly placed shot. And even if they were standing still for the time needed, chances are that you have never had to shoot a live person before and possibly end their life. People talk tough when it comes to fighting crime, but when you have to actually pull the trigger, all bets are off.
That's why I always suggest; Practicing shooting from withdrawing the pistol from your holster. No, I'm not saying that you need to practice quick draw cowboy shooting. I mean that you need to teach yourself muscle memory, so that you pull the weapon, aim the weapon, and pull off 3 shots. Practice that every time at the range. Unless it's a new gun that you're trying to break in or feel out, all practicing should be draw the weapon, aim, and pull off 3 shots. And if you're carry concealed, your mind should already be made up that if you pull out your weapon, you WILL AIM AND PULL OFF 3 SHOTS!!! You are not to think about it. You don't pull a gun out to scare someone. If you pull it out from concealed, you should be aiming and firing 3 shots. Your brain does work fast enough that if you realize that as you're pulling out the weapon, that the bad guy is turning and running away, that you won't pull the trigger. But the image you see when you're pulling the weapon, if that image is still there once the weapon is drawn, you should be firing 3 shots. If not, then there was no reason to pull the weapon in the first place. Pulling out your weapon is the last option you have. You don't walk down the street and when you see some young thugs following you, you pull the gun out and say leave me alone. Unfortunately, there are definitely going to be those who think they are going to be in a big time gang type shootout. But for the vast and overwhelming concealed carry individuals who feel that their life is indeed in danger, and they must pull out their gun, then they need to have the muscle memory in place to not think about it; unless your brain shuts your action down at the last minute. You don't have to think about that, your brain will tell you. Other than that, pull the trigger 3 times.
This training will give you the best chance of shot placement. You won't be thinking about shooting, but instead of the threat level. If you don't think about aiming, and have practiced enough, you'll get as close to perfect shot placement as possible. Truth is, if you hit them directly in the spine, and they drop like a "Sack of Spuds", it was one hell of a lucky shot. And your 3 shots is designed to increase your odds of hitting something vital, and have the bad guy's body react enough that you have a chance of getting away or shielding yourself from the threat. Remember, the purpose of shooting a bad guy is not to kill them. It's to stop a threat. if they happen to die, that's good. It means they won't have the opportunity to try and harm someone else. But your goal is to be free of the threat.