It appears that the caliber of the handgun doesn’t seem to be a factor.
I think caliber makes a difference. The sample size is too small for it to become measurable.
Even a .22 rimfire can and has been used to kill the largest bears. Where it makes a difference is the potential target size is very small and the caliber does not allow much error.
A .44 magnum allows potential body shots and can break big bones.
That difference only comes into play in a tiny number of cases.
As most people who carry handguns to protect against bears carry relatively powerful handguns, there is not a huge sample of small caliber cases to examine.
A 9mm with the best penetrating loads does pretty well. It becomes a tradeoff between controlability, number of shots, and per-shot damage.
Shot placement trumps most other things.