I can't read all the posts to this thread - or any other thread about body sizes - because, despite having completely got over my eating disorders 20 years ago, it makes me start feeling all squirmy and is dangerous. I wonder if all those of you, who've posted so knowledgeably about diet and obesity, realise how disordered you sound?
There are physical conditions which cause fatness and thinness. Lots of them, in fact, and they can change over a lifetime just like any other health condition.
Obsession with diet and body shape/image is a psychological-emotional disorder.
A physically and mentally healthy person, who accepts their body as it is and is reasonably OK with it, WILL naturally settle at a healthy weight for them.
Neither fashion and beauty editors nor the government know the correct weight & body composition for every individual in the country.
There is such a thing as puppy fat in children. Growth spurts happen in random sequence and in all directions. Fat adolescents are often on the way to becoming particularly tall or muscular young adults (this happens in my family; I know whereof I speak.)
Insufficient fat on the body leads to hormonal imbalances, interferes with the metabolism and is generally a bad thing for the health. Malnutrition is a much bigger threat than obesity.