Couple of things. Weight becomes less important at heavyweight and generally the higher up in weight you go. At heavyweight, the more skilled fighter tends to win, like Fedor, DC, Cain, etc. Also, these bigger guys Fedor fought were unskilled for the most part. Even Hunt had a lot of holes in his game when he fought Fedor which he shored up when he went on his run in the UFC, and even then he wasn't good enough to be champion. He also gave Fedor a lot of trouble despite being a lesser fighter at the time.