Hello from JustAnswer.
There is no medical condition that would cause a man to find overweight women unattractive. It is a personal and social perspective. If the situation is causing either of you to have symptoms, there may be a diagnosis related to the symptoms, such as anxiety or depression, but this would be the result of the situation, not the cause of the situation.
Even without symptoms being present, if this is creating difficulty in the relationship, then it may help to seeks counseling as a couple.