Apparently, those of us who have health insurance are fatter on average than those of us who don't.
This is a curious finding since obesity is positively correlated with poverty while health insurance is negatively correlated with poverty. In other words, the poor are more likely to be fat and are less likely to have health insurance.
So, how do we explain the study's finding that the insured are generally fatter?
Well, we know that the uninsured are often younger and healthier than their insured counterparts (which is one reason why they choose not to buy insurance). Thus, any study that simply takes a random sample from the population without controlling for age is likely to find less obesity among the younger, healthier folks who don't have health insurance. That is hardly surprising.
In short, I'm betting that the study simply failed to control for age.