Originally Posted by
merlinextraligh
The other answer, is you can simply eat more calories than you burn.
Lots of recreational cyclists delude themsleves about how many calories they burn. Lots of people don't push themselves very hard on the bike ( their riding is more akin to walking than running) and aren't burning that many calories. Then they eat a lot, justifying by all the calories they burned. Add in gatorade and power bars on the bike, and recovery drinks after, and it's a recipe for staying fat.
Then, there's the beer.
This is the truth. I've been on group rides where guys are pulling over every 20 miles for a Coke and a Snickers bar. Then at the end its pizza and beer. Generally, people grossly overestimate the calories they burn...as if cycling is some kind of magic super-activity that burns 1,000 calories every 30 minutes.
Plus, cycling is an easy sport for a fat guy. You sit on a bike and pedal. Running can destroy a fat-guys knees and is generally a lot more intense than cycling can be (meaning riding at a casual pace is a lot easier than running at a casual pace).