[bscphil]

I think it would be most accurate to say that people's bodies have a natural size given a particular food and social environment. That's a crucial bit of context that's very often neglected. Resisting one's environment is very difficult - it often requires you to have money, time, medicine, workout obsessed friends, or all of the above. Given changes in the environment in western countries over the last 50 years or so, weights are rising to their new "natural" levels as a consequence.