Inquiring minds want to know.
I always give an honest answer. I’ve never seen a pair of pants that make anyone’s butt look bigger or smaller. The most dangerous territory I’ve ventured into is “Do you like my new haircut?” Because that can make a big difference. But yeah, if I don’t like her new haircut, I’ll tell her so. Ideally, if we are a couple, she is trying to look good for me, right? So giving her anything other than my honest answer would be self-defeating. But this may be part of my whole “not understanding women” thing. Oh well.
A very useful sentence is: “Those pants don’t do much for you” . I use it all the time in the fitting booths when going shopping with friends.
So what IS the correct answer to "Do these pants make my butt look big?"
Looking at you in those pants make my pants feel uncomfortably smaller. 
“What butt?” or if it’s someone you don’t like, “Which butt?”
I always assumed the correct answer was “Hell yeah they do!”, but then I’m of the Sir Mix-a-Lot school of gluteal desireability.
The correct answer is to never get into a relationship where answering that question honestly is going to be a problem.
“No, your food/exercise balance does that.”
And then the fight started…
“'Do the pants make your butt look big? No.”
“I don’t think that’s the most flattering cut for your shape.”
Slightly more serious answer if your lady friend is not considerate enough to explicitly tell you if she’s looking for reassurance or advice:
If you’ve been around her for a while, you’ll start to notice her dressing pattern. Basically, does she try on lots of outfits, or wear the first one or two things she sees? Figure out if she’s still in the process of getting dressed, or basically done. If the former and you don’t like the pants, say something like “I think the black slacks would look better.” if the latter, as long as the pants aren’t crazy terrible, tell her she looks great.
Do women really ask this question, or is just some sitcom stereotype? I have a mirror, I know how the pants make my ass look.
Just bite down on the cyanide capsule.
It’s a trap!
Lard ass! Lard ass! Lard ass!
Compared to what?
Enough with the bread, already.
I’ll put it this way: if you ever have to haul ass, you’re gonna have to make two trips.
No, your butt makes the pants look small.
ETA amusing anecdote: Current g/f went out and bought some new jeans. She had worn one pair for half a day, and she noticed that they had stretched quit a bit, and ended up being a little big in the waist and butt. When she showed me I, without thinking, thought it would be funny to say “Well for the love of God don’t grow in to them!” I’m really glad that didn’t go over as poorly as it could have.
No, it’s the fat that makes your ass look big.
Hmmm. Ok, turn around… Hmmm. Now take the pants off and turn around.
Then, of course the trick is to…
… grab the pants and whack them against the wall viciously while proclaiming loudly, “Bad Pants! Bad…! Bad…!”
You know, I think they’ve learned their lesson…
“Your hair looks great! What have you done with it?”
“Bend over!”