The calorie counters I've seen put calorie consumption at about 500 calories/hour depending on the rider's weight and the pace they are riding. (About.com and the estimator on my Garmin are in the same ballpark). I've often wondered if that applies as much to someone that rides a lot versus someone that does not.

I eat fairly light during the week and on weekends ride a lot and eat a lot ... and the same weight mystery happens with me. I dunno if this explains it, but my theory is that the extra weight is all the food you ate, in the process of being digested. Until you expel what's left (ahem), it's going to skew your weight a bit.