It is a ratio of velocity and mass, kinetic energy. A small bullet with great velocity can do great damage, but it still needs to hit something vital and do serious damage to it. It is more about bullet placement, marksmanship. Emptying a large caliber handgun at a charging bear may do nothing to stop it, or you may get lucky and 1 bullet may hit a vital spot. If you do not take the time to aim, you are just depending on luck.

All of that said, if you have a large caliber with a very high velocity, it will likely do serious damage no matter where it hits. But it is difficult to carry a 50 caliber machine gun around.

Bullet design can increase the amount of tissue damage caused, and prevent over penetration, though with a bear in the wild over penetration is not a significant concern. If all of the kinetic energy of the bullet is dissipated into the target, that is "stopping power". Most handguns do not possess enough potential kinetic energy to stop a grizzly bear, so you must either scare it away, cause enough pain to discourage an attack, or kill it.