Matt Hart wrote something in STW a while back about how easy it is to put weight on (creeping obesity) and how hard it is to get it back off.
I don't every get on the scales because it's pretty irrelevant to me. No diets, just stuff in moderation, don't clear the kids plate, enjoy luxuries as just that, and make more of your own food (Pizza's are way nicer/healthier if you make them yourself).
If my trousers feel a bit tight, I'll eat less, that's about it 🙂 I know I'm way fitter than I was because I moved somewhere with big hills and ride as often as I can. That's the motivation for me, ride longer and faster and be able to do the same the next day.
I reckon if you didn't have a hobby that involved exercise that you enjoy (as opposed to trudging down the gym with a feeling of duty), it'd be harder to shift the flab.