Obese people race bikes. Some of them are evn fast. There was one such character in my race today. There are many folks that are simply heavy, but were it not for bike racing they'd surely looked cream filled. The truth is simply that the dynamic the OP mentions is due to the structure of races. Running and Tris have citizen type classes at the same events as the competitive categories. Folks who aren't competitive in cycling tend to do bike tours and centuries.