When I gained some weight (due to medical issues - I had been slim most of the marriage)my ex was particularly bothered and unsupportive. And I realize now that a big part of that has to do with my ex's own insecurities, and how much emphasis he puts on what other people think of him. If I wasn't perfectly slim, it was somehow a bad reflection on him.
Your ex might resent your weight loss now, because you have taken away from him the handy excuse for his behavior. Now he has to come up with another reason.
