The one obese person I work with (also a programmer) - I always see him eating terribly (candy, sugary drinks, chips etc) - the only exercise he gets is walking from his door to his car and from his car to his desk, and he's gaining weight like crazy - easily another hundred pounds since I first met him. He's big enough that when the company sent him to a convention they had to buy two plane tickets for his two seats. At lunch without fail he always picks the worse thing you could get from the various cafe's near by (like if there was a 500 calorie meal and a 1000 calorie meal - he'll always go for the 1000 calories).
I know when I was 20-30 I could eat whatever I liked and I was a skinny beanpole. After 30 though I really do have to watch what I eat - I'm guessing my metabolism slowed down. I do walk or ride my bike to work, and I used to go to the gym for about 2 hours a week (really messed up my shoulder in a bicycling accident...) - but yeah I can't sit there and snack all day without gaining serious weight anymore.
The younger guys who come to work for us - some of them eat like that and over time I've notice them get rounder and rounder as they get older :(.