Metrics don't do thirds. The 12 oz is the original, ever since Libby's of Chicago canned beef in the 1870s. And as 12oz meat was the daily soldier's ration, the rest is history.
Also bread 12, 24 oz loaves
Those old cans remind me of the time me and a buddy as got some candy bars at the little country store near our cabin. The old guy fished around for awhile and said “Here, save your money - I’ve got some left-over ten-cent bars here”. (Back when the “new” bars were 25 cents.
They smelled of mildew and tasted of kerosene (from the store’s heater). Even though we were kids, we still figured we shouldn’t eat them after the first taste!