There seems to be some confusion about gagues vs ounces of shot - and power( knockdown power, I guess ..).
Its traditional for a 12 ga to shoot 1 1/8 oz of shot - but by dropping a 12ga down to 1 oz (balistically its the same as a traditional 16ga load) - and if you drop the 12ga down to 7/8 oz then balistically its the same as a 20ga - to 3/4 oz same as a 28ga ...
7/8 oz of shot at 1200 fps coming out of a 12ga / performs and hits exactly as hard as 7/8 oz of shot at 1200 fps out of a 20ga. There is no difference in Ft-Lbs of energy / number of pellets, etc.
Personally, I like all of my hunting, sporting clays or skeet guns - in Over Unders, semi-autos - to be around 8 1/2 lbs ( even the .410 ) / so I add weight to all of my guns to get them to around the same point. The heavier the gun - for a given shell - the less recoil you get - so a little heavier gun is a good thing (if you can handle it). Yes, traditionally, most 20ga, or 28 ga etc ( were built on smaller receivers ) and were a little lighter, had shorter length of pulls, etc - but these days, gun mfg's are doing all kinds of things a little different.
I shoot a fair amount of 12ga - but I really like the 20, 28ga and the .410 as well .