The Eagle should be bigger, not because it has one oz of gold plus copper & silver because it doesn't, it is one oz total, but because the copper and silver are less dense than gold. The AGE has less gold content, so it has to be bigger to make and ounce. So I think you are right that the reason that the Buffalo is thicker is because of the design.
That is incorrect. An Eagle is 33.93 grams and a Troy oz is 31.1 grams. There is a Troy oz of gold in an Eagle.
Oh, my mistake. Didn't do the research myself as I should have.... I thought the coin was one troy ounce total metal content based on the ampex screen shot.
FYI for everyone, for reference, which I occasionally need myself. 33.931 (total weight) x .9167 (% of weight that is gold) = 31.10 total gold content, which is 1 Troy oz.