“You say that like it’s a bad thing.” seems like a perfectly acceptable response.
“No, the pants have nothing to do with it.”
If you’re looking for a serious answer, whenever my girlfriends ask me that, depending on the girl, I say: “I love you too much to tell” or: “I’m too turned on to tell.”
I’m a woman though, so YMMV.
That’s what SHE said!
Actually, your butt makes those pants look fantastic.
“You’re coming on to me, aren’t you? Well I’m up for a go.”
I went shopping with a girl friend and her friend one day. (It was still early in the relationship, and I was young, so I’d do anything to spend more time with her.)
The friend was looking for a dress, and it came down to two choices. They asked my opinion, and since I didn’t have any skin in the game, I gave my frank advice. When questioned, I honestly answered that the white one make her look flat chested. The look from the two of them was priceless.
When people are decrying big butts, what dimension are they talking about? The X axis? Or the Z axis?
“Not big enough!”
Any answer implying that the questionee finds the questioner to be unspeakably sexy is probably a good one.
I attempt to deflect the question by directing them to this discussion of the “long butt” optical illusion. Then I run.
If you don’t like the person: “No your fat butt looks big in everything” :smack:
If you like the person: You look like Pippa Middleton 
Scrubs has covered this issue, as I am coming to realize is true about most issues.
Those pants are half as big as they need to be.
Just yesterday I had the following conversation:
Girlfriend: “Does this outfit make my butt look good?”
Me: “No…your butt makes that outfit look good.”
+1 billion points for me.
I used to reply “No, honey, your butt looks big when you are naked.”
I am unmarried now.
<sigh>
My mother does. Well, she often aks the opposite: “do these pants make me look thinner?”
In moments of familiar exasperation, she’s been known to get responses such as “no, and they are way too tight” (that one was from Dad); “ok, Mom, what makes your butt look fat is the accumulation of fat, it’s got nothing to do with the pants” and “it makes you look equilibrated. You know, like your butt and your gut are about the same weight.”
We seriously have no idea why the hell does she still ask, all we know is that it still gets on our nerves.
Yeah, my answer to this question yesterday was “I dunno… turn around. Now bend over… Hang on, still thinkin’ about it… Give me a minute…”
“Let’s just say they don’t hide it”.
See, I love this question, because it’s an easy one. The answer is, “Of course not”. I know that’s the answer, and I don’t have to think about whether the pants actually look good on the person or not, or what I think they want to hear. So many of these questions in relationships are hard, because I don’t know the right answer. I’d generally rather answer a question about the theory of relativity. But this one, I know the answer to, and I don’t even have to think about it.
I suspect that actually relying on me to give anyone any useful information about whether clothes fit them well is legal evidence of insanity in most states.