Only the .410 is normally loaded heavier than the 28ga. A 7/8oz 20ga load "IS" a lighter load proportionate to gauge than is a 3/4 oz 28ga load. "If" loaded to the same shot column length in the bore a 20ga will carry 25% more shot, but have only 12% more bore contact area. Area of two cylinders of equal length are proportionate to the square of their dia's while while the area of the inner wall is proportionate to their dia's. Thus (.615/.550)² = 1.25, while .615/.550 = 1.12.
A 1oz load in a 16ga is a round ball equivelent load. It is not a square load as it's shot column length is greater than .662". According to a chart published by Hercules/Alliant in their reloaders guide it would average about .837", but they did not state shot size or alloy used. some slight variation can be expected, but not .175" in length. They calculated the volume of 1oz shot @ about .288CuIn.
Each higher numbered gauge is just a little smaller bore than the last one. There is absolutely no "Magic" involved in any of the sizes.
If you measure most "Modern" plastic wads in comparsion with the older wads, i believe you will find they "Are Smaller", in fact smaller than a "Normal" bore of the gauge. They all, at least all that I have dealt with require obturation of the base section for sealing. I take most of those "Overbore" claims with a big dose of salt.
As Sgt Joe Friday used to say " Just the Facts Ma'am, Nothing but the "Facts""

Miller/TN
I Didn't Say Everything I Said, Yogi Berra