I think is IS a form of bonding, but I also have always felt it's very disingenuous. My interpretation was always that the person who says something like "my butt looks so big" doesn't really think that's true; she is looking for compliments or validation. To respond with a competing comment is ignoring the first woman's need for a compliment but subtly implies that she's accurate. It seems like a very catty kind of exchange.
I remember this kind of fat talk from high school, but the only person I've heard talk like this in the past 30 years is my mother! My children are all boys, so I have been sheltered to how young this starts and how widespread. Very interesting study.