“Look, Honey, you know you have a fat ass, I know you have a fat ass and the world knows you have a fat ass. Do you really need remineded of this fact? Now hurry up I want to get some pizza.”
Well, in all seriousness, my wife has a big butt. She knows she has a big butt. I* like *her big butt, and I cannot lie.
Not long ago I was in the fitting room with her, helping her pick out some pants. You see, certain cuts, colors, fits, and so forth help give her a luciously bodacious badonkadonk. Others are, shall we say, not so flattering. She trusts my judgement about which is which.
So it is in fact possile to be asked that question, provide honest feedback, and *not *fear for one’s life. 
Well, according to James P. Connolly, the correct answer is:
“Of course not! But those are some big damn pants.”
If she’s shopping for new pants and genuinely looking for advice, be honest. Tell her the pants look good or the pants don’t suit her – leave the butt in question out of it. There is a pair of pants for every keister, but it may take a lot of looking to find them, and it really is kind of difficult to get a good look at your own backside, even in dressing room mirrors.
If she already owns the pants and is getting dressed for something, especially if she asks you more than once, she hates the pants (or her butt, or the way her butt looks in the pants, or the event you’re going to, or her coworkers, or the price of tea in China, etc etc etc) and no matter what you’ll answer, she’s not going to change her mind. So in the interest of keeping yourself out of trouble, I’d try something like, “Gosh, honey, I’ve been staring at your butt all day, and it looks fine to me.” She may throw something at you, but it’s more likely to be the pants than, say, a vase. 
What I recommend using back to your “lovey-dovey one” should be: **You say back to her **(take charge of the conversation), “Let’s state this better, baby.” “Ask me, 'Do you like how my ass looks?”’ You tell her that her ass looks beautiful and absolutely gorgeous. “Oh yeah, I Love those buns!!!” I think this will spice things up, after you “playfully” start-in on some serious smooching on that ass; with some sweet caressing and spankings. Don’t get too carried away. They’ll love it and they’ll ask a lot more, plus, now you’ll have the correct reply. It works like a charm (when the mood is right). Remember, don’t start an argument. you’ll never win. I always use “tender sexy whispers” in her ear that she looks so hot. This works too. Always use simple sincere compliments to the one you love. And also, never look at other women. Ever. She only wants your attention.
Spank that phat zombie ass!
“Do these pants make my butt look big?” = “I like to fight!”
Correct reply–“I find you so arousing, right now!” Hold her close, & purr in her ear.
Gosh, it’s hard to argue with that.
… and then the murders began.
(Wait, is this the right thread?
“Do these pants make my butt look big?”
Third worst possible response:
“Yes, but your tits make up for it.”
Second worst possible response:
“No, it’s not the pants that are the problem.”
Worst possible response:
“Yes, but they would look great on your sister.”
Almost six years later, gotta ask the OP: Need answer fast?
My ex replied “compared to what? An elephant? The SST?” So happy he left me.