This thread is going to be full of "offend"...
Cycling is easy. You have to try if you want to get a workout on the bike. Ever see those same potbelly dudes in the drops hammering on the flats, or crushing it in the hills? I don't. Most of the fat riders I see are soft pedaling (with legs akimbo) on non-hilly roads. Don't get me wrong, there definitely are some bigger riders who try hard and want to lose weight, but cycling is something you can do for an hour or so and make yourself feel like you are exercising.