I feel the % increase in British guns is less because Brit guns were usually upland guns and lighter than their American cousins. In our country, 12's (and 10's) were all around guns for deer and waterfowl while sub-gauges were for upland game. In Britain, 12's were mainly upland guns and 6 pounders were not uncommon. A 20 in a classic American shotgun is both rare and neatly balanced and light. They are worth more and sell quickly.
The Brits were much more into "purpose built" guns, while American gunmakers turned out far more "all around" guns. There's no shortage of vintage British fowlers and pigeon guns. We certainly had trap guns and waterfowl guns, but a lot of Americans used their Sterlingworth 12's for everything. I look at most American 12's in the same way I view a pair of the old LL Bean boots: not perfect for any specific purpose, but functional for just about everything. And while there are plenty of 12ga Brit game guns in the 6 1/4-6 3/4# range, finding one much lighter than that takes a fair amount of looking. But it takes a whole lot more looking to find an American 12 that goes mid-6#--which is why the 16 found (and still finds) favor with those who are looking for a gun for upland game.
