Anyway, the size differences are in weight, but you can also tell by the body shape. Bantams are more compact and the standard sized ones have a more wild nature and look to them with longer limbs and tails.
As far as weight differences between the two, Large American Gamefowl vary greatly by strain, I'd guess just under 5 pounds on the low end for cocks on up.
AGBs are supposed to be 30 ounces, but tend to range a bit larger, so 2 pounds, +/-. AGBs are more standardized in terms of conformation, toe and shank color, etc., as they are an accepted APA and ABA breed.