Back in the day, the heaviest loads our North American ammunition companies offered in the "standard" 2 1/2 inch 20-gauge shell was 2 1/4 drams of bulk smokeless powder (or 18 grains of dense smokeless powders such as Infallible or Ballistite) pushing 7/8 ounce of shot. In the longer 2 3/4, 2 7/8 and 3-inch 20-gauge shells they offered 2 1/2 drams of bulk smokeless powder (or 20 grains of dense smokeless powders such as Infallible or Ballistite) pushing that same 7/8 ounce of shot.
Arrow Rem-UMC 20-gauge 3-inch Label Load.jpeg
The perceived advantage of the 2 7/8 inch and 3-inch shells was more/better wadding. Lots of the commonly used 20-gauge shells were lighter like 2 drams pushing 3/4 ounce of shot.
Nitro Club 20-gauge 01.jpg