Willyumyum has the reasoning as me, except I was a fat ass until I was 32. Then I decided that the hell with dropping dead at 40 (high stress job, high blood pressure).
Bought a bicycle and decided that I was going to make some changes! Went from 250lbs, 46"waist (i am 6'1") down to 165lbs 30"waist over the course of a year.
Felt much better, blood pressure went from 160/90 down to 115/80 with a resting pulse rate of 50.
Then I decied that I was too damn skinny. Over the next 3 years managed to get to 210lbs, 32" waist naturally. Folks really started to notice then. I felt great, but wanted more. So a year ago I decided go for it. Now up to 240lbs, but waist creept up to 35", but I can fix that in the spring.
I figure another 20-30 lbs and I will be happy.
Funny thing is that people tell me I am huge, and I really dont see it. That I will need to reconcile for my self.