Must resist urges to throttle.....
The 12 is where the lines cross on the graph between portability and effectiveness, using commonly available ammunition.
Sportin' gennulmen of yesteryear oft toted an 8 or 10 gauge for waterfowl, a 16 for upland game. We coalesced both missions into a single tool and that turned out to be the 12 gauge.
Need an oz or so of shot for smaller birds? The 12 will do that.
Need a teacup of 2s for big geese, cranes, pterodactyls? The 12 will do that also.
The 12 may be a little heavy for upland work or kick a bit too hard with the barnburner loads, but ti's a compromise that does work.
The standard 2 3/4 12 gauge case will hold enough lead shot for most missions or a .73 caliber slug suitable for most game stuff under a ton. The longer cases do the same for non toxics.
S'funny, most of the shooting I do these days is at clays, and with 7/8 oz loads. 7/8 oz used to be 20 gauge country....