I imagine a lot of women go through this--because my sense would be that women tend to a) gain cellulite when men really don't, b) have children who stretch out their bodies and often instigate weight gain, and c) work in non-physical jobs and do the majority of work in the home (which means, less time at the gym--more time around the food.)
Plus, the media generally portrays (extremely narrowly defined)beauty as necessity for women, whereas men can be valued as attractive for character, despite their age, weight etc.
Anyhoo--
I have trouble with my S/O because although I am really not too much bigger than him, I am not an athlete. Since we've been together, I've put on 30 lbs. He hasn't so much--in fact, he goes to the gym for an hour every lunch. The thing about it is that he really loves activity--its actually fun for him! Whereas, I like writing, art, museums...I have to force myself to do exercise.
It isn't such a problem, but sometimes I get jealous of the people he hangs out with. He went on this skiing trip recently, it was just him, a couple that we both know, and two women that I had never met. These people are the kind of people who spend every waking moment of leisure planning out or doing marathons, cross-country skiing, backpacking trips, mountain biking, surfing, mountain climbing, wind surfing, scuba diving, white-water rafting, caving, racing bikes etc, etc, etc. Exercise and athletics is, in short, their life.
I could never compare to that. So I end up kind of insecure.
One of the solutions I entertain is for me to get a punching bag, and think of all the perfectly athletic women!!

That could give me a good work out. I am kidding of course; jealousy is such a strange teacher.