Not sure I really classify Kemo as a big butt, or a bust. Maybe his boiler overshadowed his posterior, but I think he had a large frame and width, but not volume of gluteus. He didn't even have the largest butt in his draft class, as I think Trai Essex beats him there. Also for a 6th round pick to start and play 7 NFL seasons, I cant call him a bust.
I think we have to mention Max Starks. Later in his career he packed on some weight I think, where as his early years he was a bit more trim. Kind of like young Elvis Pressley vs Old Elvis.
Also, Chris Scott from Tennessee might get mention, but I don't know if he ever made a 53 man roster, but I seem to remember him having some junk in the trunk.