Most combat sports like boxing, wrestling, etc. have weight classes because they recognize that people have different builds/sizes. None of these athletes are fat, but are taller/shorter, more muscular/skinnier, etc. Running is obviously not a combat sport, but weight is obviously a factor. The runners built like ectomorphs (tall and skinny) often have the upper hand. This is the opposite of boxing, etc. but the same concept applies: weight can make a difference. How about instead of worrying about losing weight (eating disorders too), we train hard and race at our natural weight against others built like us?
This is an example of the boxing weight classes:
http://boxrec.com/media/index.php/Weight_divisions
Maybe running wouldn't need this many, but I'm thinking something like 10-20 pound increments.