The "big difference" in "modern" progressive burning powder shells (I'll use the 12-gauge example) is that they pushed the 1 1/4 ounce payload at a higher velocity, with an equal or perhaps a bit lower pressure, than the older bulk or dense smokeless powder loads could. Or, the progressive burning powders could be used to push a heavier payload, ie the 1 3/8 ounce 12-gauge Super-X load in Western Cartridge Co.'s 3-inch Record shell, at a equal or even slightly higher velocity.
Western Cartridge Co. had led the way in introducing progressive burning smokeless powder to shotgun shells with the 1922 introduction of their Super-X loads in 2 3/4 inch 12- and 20-gauge Field shells.
The next year they added a 16-gauge Super-X load in their 2 9/16 inch Field shell, and a bit later the 12-gauge 3-inch Super-X load put up in their Record shell.
By 1926, Western had added the .410-bore Super-X load and had breathed new life in the moribund 10-gauge by introducing the 2 7/8 inch Super-Ten load with 1 5/8 ounces of shot, a terrific increase over the previous maximum 10-gauge loads of 4 1/4 drams of bulk smokeless powder and 1 1/4 ounces of shot or 32 grains of dense smokeless powder, such as Ballistite or Infallible, and 1 1/4 ounces of shot!! By this time only Parker Bros. and Ithaca Gun Co. were offering 10-gauge guns in North America. About 1928, Western Cartridge Co. introduced their copper plated Lubaloy shot. The Lubaloy Super-X loads were all put up in their high brass Record shell.
The first smokeless powder for shotgun shells was probably Wood powder, introduced in 1876. Shotgunners being a hidebound lot were rather slow to embrace smokeless powder, but by the 1890s it was coming on strong. In 1890, Captain A.W. Money came to America from England, and established the American E.C. and Schultze Powder Company in Oakland Park, Bergen County, New Jersey, with offices on Broadway in New York City, to manufacture smokeless shotgun powders. In 1893, Union Metallic Cartridge Co. was already offering smokeless powder shotshells, and that year Winchester was providing them to selected shooters with Winchester offering them to the general public in 1894. The American ammunition companies held their smokeless powder loads offered in the 2 5/8 inch 12-gauge shells lower than those offered in the 2 3/4 inch and longer shells. Same thing holds for the 2 9/16 inch 16-gauge shells and the 2 ½ inch 20-gauge shells. The very heaviest 2 5/8 inch shells I find offered were 3 1/4 drams of bulk smokeless powder or 26 grains of dense smokeless powders such as Ballistite or Infallible with 1 1/4 ounces of shot. In 2 3/4 inch and longer shells they offered 3 1/2 drams of bulk smokeless powders or 28 grains of Ballistite or Infallible dense smokeless powders with the same 1 1/4 ounce of shot. These loads were very high pressure according to a DuPont Smokeless Shotgun Powders (1933) book I have. It shows the 3 1/2 drams of DuPont bulk smokeless powder pushing 1 1/4 ounces of shot as being 11,700 pounds; 3 1/2 drams of Schultze bulk smokeless powders pushing 1 1/4 ounces of shot being 11,800 pounds and the 28-grains of Ballistite pushing the 1 1/4 ounces of shot being 12,600 pounds!!! There were plenty of lighter loads being offered, but American shotgunners being what they are, I'm sure many were opting for the heaviest loads available. The same situation held with the 16- and 20-gauge shells. The "standard" 2 1/2 inch 20-gauge shells and the "standard" 2 9/16 inch 16-gauge shells carried slightly milder loads than the extra cost longer shells in 2 3/4, 2 7/8, and 3-inch lengths.
Many folks believe that the "modern" shotshells loaded with progressive burning smokeless powders, introduced in the early 1920s, Western Cartridge Company's Super-X loads leading the way, were higher pressure than the old bulk and dense smokeless powder loads. Reading period literature, this is not the case. With progressive burning smokeless powders they were able to move out equal shot loads at higher velocity or a heavier shot load at equal velocity, but at lower pressure than the old style bulk or dense smokeless powders.