Yes, I know you all winced, dove for cover, or ran upon reading that. Come back, or fail to learn the way to escape this ambush!
First, take a deep, calming breath. No, go get a beer. That'll help overcome your aversion therapy more. This is one ambush where trying to avoid answering is the wrong answer. (As you already know.)
Do NOT focus upon the female assets and their fatness or lack thereof. (I know, I hear you cry you already learned that, and are showing each other the scars to prove it.) DO FOCUS on the Article Of Clothing. Make sure you are noticed to be scrutinizing the article of clothing.
Examine the article of clothing as if it exists totally independent of the female body part: the ass does not exist. Compare it to other articles of clothing. Then pronounce judgement upon the article of clothing, and upon it and it alone. Do not show fear. Do not hesitate. Who dares, wins.
For example, for an already-bought pair of pants that do not look bad: "Do these pants make my ass look fat?" "Hmm. No, I think they look fine."
For an article of clothing being tried on at the store, prior to buying, that is Not Good. "Do these pants make my ass look fat?" "Hmm. Those particular pants are not very flattering. Don't worry, dear; I know you'll find something you like."
For an already-bought article of clothing that is Not Good: "Does these pants make my ass look fat?" "I know you love them, dear, but I think it's time to retire them. Why don't you try the black pair instead with that outfit?"
Understand, there are times that ever The Right Answer will not save you - and that's because you have encountered a moment when, instead of taking you by the hand and leading you into a minefield all unthinking, she actually, truly, wants to have a fight - and this was her declaration of war. Any accusatory response that comes out of her mouth in response is likely to be a non-issue, a distracting flanking attack while she readies her main verbal guns.
That's when it's time for "I love you, dear. What's really wrong, and how can we fix it together?"
Now excuse me, I need to go ready the mini-gun and double-check my fields of fire for the femininity that may want revenge at the spilling of their secrets. They scratch and bite; I shoot. Don't worry, I'll be back.