Keep in mind that when a shotshell is fired pressure fills the Whole barrel. Maximum or peak pressure is definitely not proportional to payload/velocity. Total pressure is very close to it. There are a few other factors which enter the picture which prevents it being exactly proportional. One such factor is difference in bore contact. If loads were exactly proportional to bore size they would have identical column lengths in the bore & weight would be proportional to the square of the bore diameter, but bore contact area would be proportional to the diameters. Thus if we compared a .410 & 20 which had the same column length of shot the 20 would have a 2.25 heavier charge weight but only 1.5 times more bore contact area. thus the friction of getting the shot down the barrel would be higher in the .410 which would produce a slightly higher total pressure level. "Peak" pressure can of course be varied by changing the burn rate of the powder used in either case. I still find nothing amazing that small bores are normally loaded to a higher pressure level, it's just the nature of the beasts as they say.
