Ooooh, what'd I do? :redface: Only part of my prior post went up.
The part that's missing was something like;
While both the 20 and 12 may both be able to shoot a 1 1/4 oz. load, the 12 would most likely give a larger diameter on the patern board than the 20. But, the 20 would have a longer string of shot going to the board. With larger shot, this may lead to a thinner patern of shot on a pass-thru / swing thru situation. Also, the 12 can be (shot)cupped to give the same shot density as the 20 with a significantly shorter shot string, which in essence, can give the same shot density of a 20 at greater range (the 20 just doesn't have that (range) capability. Or the 12 can be loaded to string and patern much more successfully to the 20 than vise-versa. This would (in some opinions) make the 12 more desirable for all around squerrel, rabbit, pheasant, duck, and goose (etc.) hunting.
At least this is what I've learned fron the local "Sporting Clays" and other crowds as to why they shoot the loads they do.
And it sounds probable to me but I can't say I can prove it.
PS; I do hear a lot of controvery over "if" stringing shot is an vertue or burden. I'm thinking it depends on the application. The situation of a duck jumping and going away may find a short stinged, heavier shot, long range cupped shot as a plus. While a flushing dog that jumps a quail going across your path of sight may be better suited to a longer sting of less compact shot at shorter range thereby giving more latitude with your quick "lead" estimate.
Well, that's sorta how it was explained to me. :v