Actually, I have run across a few fat guys that can really ride...especially on the flats where their weight is less of a detriment. I do try to test them in the hills however.
Believe the point to the OP is...having blub around the mid section and riding a small bike for your size as maybe a top racer would with shortish headtube is not a recommended formula. To me the worse combination of fit is small bike + short cockpit...double kiss of death. Fat guys should ride a bit more upright to keep from self flagellation. But if developing a higher pain threshold to ride faster, this maybe the best formula.