I have had many patients over the years who have gained weight as a result of medical conditions. Believe it or not there are so many causes of uncontrollable weight gain, such as liver toxicity, food allergies (which leads to great water retention), carbohydrate intolerance, which leads to excess insulin secretion, which, in turn causes fat storage and inability to lose weight, low thyroid, hormone imbalances, and even excess cortisol (cortisol is secreted when a person is stressed) production, which can lead to massive water retention and weight gain.
My point is, if your partner had gained weight due to a medical condition, you would probably also feel a diminishment in your level of attraction, and a sense of loss; but, in your case, it sounds like you believe that your wife's weight problem isn't out of her control. Rather, it seems that you are saying that she simply doesn't care about her weight or your feelings about her body. In other words, your wife is telling you through her behavior that she doesn't care to be responsive to your needs. In other words, her weight is a symptom of a relational or interactive problem between the two of you.
Her refusal to do something about her weight is another way of saying, 'screw you. . . I don't care about how you feel. ' In reality your issue isn't her weight, it's really about how she disses you and your feelings. It's about how she indirectly expresses her anger toward you by ignoring your feelings. She is sticking it to you through her body, somebody else might send the same message by overspending or ignoring you.
Rather than broach the subject of her weight head on, I think you need to start talking about what her behavior is actually saying to you. You need to help her to talk about why she is so angry at you that she doesn't want to be responsive to your feelings. If you can get her talking about her feelings of anger toward you, there is a good chance that she will no longer need to punish you with her body. Let me know how you make out.