As a teacher who worked with people with special needs, including those with obesity, I think it is time to redress the general level of ignorance, superstition and bias so far displayed in this thread:
At an individual level, a combination of excessive food energy intake and a lack of physical activity is thought to explain most cases of obesity. A limited number of cases are due primarily to genetics, medical reasons, or psychiatric illness. In contrast, increasing rates of obesity at a societal level are felt to be due to an easily accessible and palatable diet, increased reliance on cars, and mechanized manufacturing
(From Wiki)
Note in particular the mention in the last line of the modern diet. It is a sad but true fact that children become obese at an early age because they are given, for any number of reasons, an inappropriate diet. By the time they reach their teens, their habits and size are pretty well determined and after that it is very difficult to undo the damage. It is easy in this scenario to blame parents but the fact is that many people in today's society do not have sufficient knowledge or education to understand what constitutes a healthy diet for their children. Hence the attempts by Jamie Oliver to try to get a healthy diet into schools. All of this is exacerbated by the processed food conglomerates who are able to produce inappropriate packaged food very cheaply and make vast profits.
You may well be discomfited by the obese person in the seat next to you but please do try to be a little less prejudiced when discussing such a situation. By the way, should you wonder, I am fortunate to have been slim all my life thanks to the fact that my mum brought me up on a very healthy wartime diet.