A few criteria to consider. Make your own conclusions.
For defensive stopping power:
You want to inflict the maximum damage in as few bullets as possible. Consider hollow points.
This is done (generally) with the highest caliber allowed in your area, such as .45acp, or 357 magnum.
However, to be effective, it must be a caliber that the shooter has full mastery and practice. A highly lethal caliber bullet is useless in the hands of a person who cannot control it.
Example 1): if an 357 magnum revolver hurts your wrist, flies out of your hand, and the shots are scattered, consider a lower caliber such as 38+P. Keep going lower in caliber until you can fully control it and regularly hit bullseye at 15 ft.
Example 2): a humble .22 that hits the threat target in the center mass, is obviously more effective than a hand cannon that misses every shot. Remember the Divine Intervention scene in Pulp Fiction? The shooter had a hand cannon but no experience handling it, thus missing every shot.
Find the biggest caliber that creates stopping power, but you must control the beast.
Next criteria is gun weight. For defensive carry lightweight, it often ends up being a snub nose short barrel revolver or pistol. Ruger LCR or LCP. Glock also has a respectable line.
Then next criteria item is the ammo capacity of the gun, the legality of max capacity in your area, and how much more ammo you need to carry, and how fast you need to reload.
Each person can assess their risk situation.
Example: animal threat, such as a Pack of 5 wolves, 3 hungry bears (2 cubs 1 mama bear), 2 very bad humans, or 1 moose in mating season. All those are rare but plausible scenarios in certain areas.
Based on your skill comfort with your weapon, and your caliber, what is the maximum number of consecutive shots you need to neutralize a threat.
How fast are you reloading under stress, adrenalin, panic, etc?
From my experience, I expect to need no more than 5 rounds accurately placed to neutralize a threat. The rounds that I am comfortable delivering are high caliber, hollow points, but that is a result of much practice to master the skill.
So to reduce weight, be proficient at the highest caliber you can master, then you will not need to carry additional ammo.