By bike commuting, you are burning a lot more calories per day than the average person. You need to replace those calories, so you need to eat more than I do just to maintain your current weight. Granted, it makes a lot more sense to burn the calories actually accomplishing something than going to the gym. But if the average person ate more calories than you do when you're burning off several hundred extra calories per day, they'd all look like Honey Boo Boo's mom... oh wait, the average person DOES look like than now, don't they?
(My daughter is a cheerleader. I have to buy her 3 full size cheeseburgers and a milkshake after every practice... she's frickin' 12 years old and weighs about 80 pounds, and she still eats twice as much as I do at more than twice her weight!)