Women seem to have a very serious self image problem. She can have the most beautiful body in the world. She can have enough men in her entourage to start a small ground war. Still, there's a very good chance that she'll think that she's fat.

She is very vocal about this too.
"I'm depressed, I'm fat",
"I'm getting fat",
"Look at all this fat!!" (as she pulls up her shirt to reveal a perfect stomach)

She can be told a thousand times that she's not. A supermodel could come out of a limo and tell her that she wishes she had her body and she will still focus like a laser on that time last year when she went home for a holiday and her sister told her that her arms had gotten a little bigger.

She may say something about how she was thinner in high school. Nearly everyone was thinner in high school. Often times sickly thin. Most guys don't want a girl that they can grab around the waist with one hand and do arm curls with. Just because someone weighs a little more now doesn't mean that they have become fat.

There are a few reasons I can think of why a woman would complain about being fat when she's not:
1. She is insecure and wants to be told that she is not fat to reassure herself. This is probably the most common reason. Basically, she wants to make herself feel better at the expense of annoying some local male. She can solve this by growing an ego. Not easy, but doable.
2. She is doing it to attract attention to her body because she knows she's beautiful. If she is actually good looking and doesn't mind me staring at her body, I don't mind this too much. However, it should probably be prevented as to avoid falling into category 3.
3. She actually believes she's fat. She's probably used to be in category 1 or 2 but has called herself fat so many times that she actually believes it. It's been repeated so often for her own selfish reasons that it has become a permanent fixture in her brain. This too can be fixed, but it would take much time and would probably never actually happen.

However, in all its futility, I will try this one more time. Maybe one of you out there will actually believe it. You AREN'T fat, damnit!. I don't lie when I say it, those that are fat tend to not ask. But I wouldn't lie to them either if they did. Now please find a better thing to whine about, like how I can't read your mind.