Well, there are a few large trials riders, so it's not all down to laziness. But then, those guys are fit despite being large so they don't really come under the negative meaning of a fat person. It's sometimes genetic, but the majority of fat people just eat way too much and move way too little. A morbidly obese tax would be a good idea, it'd be easy to enforce it as well. Policemen wouldn't have a problem with spotting the culprits on the street eh?