Being overweight is not a sin, no matter what the circumstances. It cannot be a sin, as being overweight is never an act, but a condition.
In the same way, things like being an unwed mother or having cirrhosis of the liver are not sins, because they too are conditions, not acts. It is possible that the sinful act of repeated gluttony led to the condition of obesity or the sinful act of pre-martial sex led to the condition of being an unwed mother, or the sin of excessive alcohol consumption led to the condition of cirrhosis of the liver—but in all those cases, the sin is the act, not the condition.
I hate to harp on this point, but I think the distinction matters because if we do not make this distinction we are acting like people can be in a perpetual state of sin for the commission of one (or even many) of these acts, and that is not the case. For all we know, the obese person may have confessed gluttony, the alcoholic may have confessed his alcohol abuse, the unwed mother may have confessed pre-martial sex and so on.