There's this one stunt he did I can't find in gif form but he puts one leg up on the piano, then he like picks up his other leg and puts it on the piano and he seems to hold it a bit before collapsing. I tried to do that all day when I was a kid and it just seemed impossible to me. I have no idea what kind of muscles you need to be able to pull that off.

See there is a difference between show muscles and functional muscles. Quite often those big beefcake looking guys have a lot of big useless muscle that is not actually functional for anything. Then you can have a guy with wirery functional muscle, they may not look as strong but they are actually stronger than the big dumb show muscle guy. Buster Keaton was all functional muscle.