A good shooter can reload pretty quickly when a gun goes dry—and they all can go dry. If you carry a double-stack high-capacity handgun and two spare magazines, you will have a lot more ammo with you. For example, a Glock 17 9mm holds 17 rounds in each magazine. So with three mags (one in the gun, two on the belt) and a cartridge in the chamber, that’s 52 rounds. A 1911 holds eight rounds in the magazine, so with two extra mags, you will have 25 rounds with you.
There are, of course, lots of double-stack .45 ACP handguns out there. They don’t have quite the capacity of the 9mm wonder-guns, but the effective striking power of the .45 counts for a lot anytime you are shooting to save your life.