Butterbean... Juan Diaz and Johnny Tapia weren't really fat but they never looked like they were in shape either so I think they get mention. Maybe Buster Mathis too
old roberto duran doesn't count? I guess he was never really that fat in the ring ... I go with James Toney, there's a middleweight hiding under there somewhere beating up heavyweights.
Big George, though I don't consider him to be truly fat except in a few fights. After that, Toney. Butterbean was the king of the four rounders....but real fights are at least 10. To me he's still a circus act.