I think it's also a matter of self-deprecation being the "social norm" for women. It's considered not only acceptable, but even humble for women to put themselves down, and I think they often do it without consideration for those around them. While it's true they may be fishing for a compliment by whining about how fat/ugly they supposedly are, I think more often than not women are simply conditioned to hate their own bodies and have an uncontrollable need to share.
I recall a conversation a coworker and I had several years ago. My weight loss of 20 pounds at the time (from 360 to 340) had prompted her to complain about the 20 pounds she had gained since she'd been in high school (not sure of her weight but we were about the the same height and she was by no means obese). I pointed out how she was a lot smaller than me and had so much less to lose, and she quickly pointed out that I carried my weight "well" and that, unlike me, she wasn't "used" to being fat! She also complimented me for having the stamina to lose the 20 pounds that she herself was struggling with; it was hurtful at the time and really hard to wrap my head around, but eventually made me realize we all have different standards for ourselves that we don't even think about applying to others when we discuss weight loss and how we fell about our own bodies.