You don't have to support your own body weight cycling. Thus excess weight is not as big a penalty cycling as it is in other sports.

Excess weight obviously hurts climbing, but it's a much smaller penalty on the flats, where power to surface volume is more important than power to weight (and surface volume doesn't increase proportionately with weight.

Hence, fat, but strong, guys like me can win races that don't involve significant climbing.

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.