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Ahhhh....that is where the sexes divide and this you must understand clearly.  If you do, it may help you in the future.  Dressing room mirrors are the worst in the whole world!!! They are too close to you, for one thing, and they are never flattering because they are the cheapest mirrors money can buy.  
Your wife already knows if she is "fat" or not, she is looking for confirmation that youstill  find her desirable even though she is fat, particularly in THAT outfit.  If you do, she wants to keep that outfit because looking attractive to you because she is wearing that outfit is a heck of a lot cheaper and easier than dieting until she is no longer, in her mind, too fat.
Just say, "No, dear, you look lovely to me~always!"  
This will make your life easier and your wife happier about who she is right now, standing on the earth in her own skin, knowing she has changed but still needing to know you find her attractive.
Or, if you don't care for the outfit, something like, "You are always attractive to me, but I've always loved your ____(name of some other clothing article that looks particularly nice on her)_____; it brings out the lovely colour of your eyes (or something similar).