Hmm, one could also have a small or regular capacity ammunition feeding device?

But your point, I didn’t initially consider they would use that to mean magazine, I guessed they meant the gizmo one might use to feed ammo rapidly into a magazine without getting one’s thumb tired, sometimes called a speed loader.

Seems silly to call it something else, but perhaps they needed a term that was a superset of magazine and anything else someone might come up with to hold more than a regular mag.

Standard capacity magazines are routinely referred to as "high-capacity," usually by people who want to ban them.

Or a drum (technically a magazine of course but certainly not what "ships" with new firearms, not what most picture when they hear magazine).