A recent study showed that having health insurance makes people fatter. The cause seems to be that people are more willing to 'let themselves go' if they know that someone else will be paying all or part of the resulting health bills.
Economists are constantly finding evidence of behavior like this. This is proof that people can and will change their behavior in response to cash incentives. Being obese is not just a matter of culture or genetics, it happens as a result of choices, and people will change their choices when the environment changes.
If you are not an economist, you would assume that health insurance would make you thinner. You would think that being insured would mean more visits to doctors who would work to get you to change your behavior. You would think that education, social pressure, and preventative treatment would reduce obesity. But that is not happening.
We will continue to have problems with obesity unless incentives are changed to improve people's behavior. Right now, we seem to be headed in the wrong direction.