Alright, I'm sure what follows is going to get people who haven't done much shooting and killing with handguns all riled up...
You have to hit vitals first, and by vitals I mean CNS. Being extremely generous- maybe an 8in target. And they only reason I say that is because it's been my general experience that when things get shot in the face multiple times they tend to leave you alone, even if not hit in the CNS.
Then the bullet has to penetrate deep enough to reach those vitals. There is no magic with pistol bullets. People shoot bears with 375's and 458's and they run off, attack the hunters, etc. There isn't a revolver or pistol that can be carried made that will touch a rifle cartridge for damage in tissue. The people that on one hand say if they had to stop a charging bear want a 375 or 458 and on the other hand think a 44mag or 454 Casull are "POWERFUL" are out of their minds.
As to wound size- if you shot an animal with a 44mag with hardcast bullet, a 45 auto with hardcast, and a 9mm with hardcast bullet, you'd be hard pressed to point out which was which. Wound channel width will be nearly the same between them. So the question is can a 9mm/40/45 with hardcast penetrate deep enough? Yes, and from alll angles. The Buffalo Bore hardcast +P 9mm rounds are ridiculous diggers and they can be CONTROLLED.
That brings us to actually hitting with a handgun during what for most will be an extremely stressful moment, probably by surprise and at a rapidly moving target. I've seen quite a few people that claim to be able to rock and roll with the big bore revolvers.... None of them could shoot fast enough and accurate enough to be using them for defense in my opinion. That includes some from Gunsight that teach big bore courses. Even reletivly trained shooters are looking at 2-3 rounds in 3 seconds on a 8in target at 7 yards with the big bores. With the same level of trainingi and practice a person can double or triple that with a regular service pistol.
If I absolutely knew that I was going to be attacked by a bear tomorrow and the only thing I could have is a pistol, I'll take a compensated, dot sighted 9mm with BB Hardcast +P's.