Because it's just not true. There have always been huge guys in the sport. Lesnar is essentially Mark Kerr 2.0, and Carwin and Rogers are like 2010 versions of Paul Varelans. Hell, Bob Sapp fought Nogueira at like 375 pounds almost 10 years ago. Tim Sylvia was 6'8", cutting to 265 and champion 7 years ago. Pedro Rizzo was 245. Frank Mir was 250ish 8 years ago. Ricco Rodriguez was like 250 and champion. So was Josh Barnett. This is nothing new. Yet would you be really surprised if a guy like Jon Jones threw them all on their head? I wouldn't bat an eye. Big HWs are clumsy and unskilled. Who cares.
And Fedor didn't lose because Bigfoot is big. He lost because his defensive grappling was terrible and he had zero gameplan.