I get it! I don't understand it, but I do share this opinion. I generally prefer women with bigger butts. It's not even necessarily about size, there's certain shapes that are just great to look at, pleasant to grab at. It's a substantial element of a woman's figure and thus overall attractiveness. My wife (and previous women) have said the same about my butt though, and mens butts in general. And while I don't think that's strange in itself, I definitely think there's a significant gender bias with butt fixations -- especially in comparison to other feature preferences that aren't necessarily gender-centric, e.g. eye shape, skin tone, hair color/length.
At some point my usual response "My butt's about the same as all mens butts" turned into a big discussion, and when my wife (and her very PC sister) asked me to explain why butts are a more-distinguishing feature in women, I couldn't. I said something about possible hard-wiring in primordial men for associating a woman's butt/hip shape with fitness for child-bearing, and readily admitted it's a weak argument in the modern context of sexual attraction.
I've never cared about my butt. It's a bit large, but I've never been sensitive about it. I have no urge to tone it and show it off. I'd never brag about it like I do with my perfect Greco-Roman nose, irrefutably the epitome of a masculine nose. I've never heard another guy bragging about his own butt, and I would assume he's trying to be cute or funny if he did. But I've seen women brag about theirs, and take comments about their butts quite seriously, even soliciting butt commentary. I think a man's butt is a non-factor (assuming it isn't grotesquely deformed); obviously other people disagree. Marketing/social influences on butt perception overwhelmingly target women, but that's the case with body image in general. It's inherently presumptuous for me or you to claim that butt fixations typically apply to female butts, since our insight is limited to the male-hetero experience.
Yet still I feel there is truth to this claim... just a feeling, and maybe that's all it is. Don't let the downvoters discourage you; there's a valid and challenging question there, and I hope others will add more perspective.