The bigger the cartridge, the bigger the gun. The bigger the gun, the worse fit it is for our hands. Add to the mix a double-stack magazine, more of these bigger cartridges, and you have a handful and more. This can be a problem, especially in the day of small stature police officers and military types. The solution is a smaller caliber, hence a smaller gun, but then you do without the vaunted power of the .45ACP. If you want a light .45ACP that carries lots of ammo, you can dream on.
Leave it to the crew at Springfield Armory to come up with a solution to this problem. Their XD (“X-treme Duty”) in .45GAP was “all but ACP” in power with the size of the existing XD. It was also a 9-round auto. To get 13 rounds, you needed a smaller cartridge.