Despite what the scale says, do you see any difference in how your clothes fit? When I started riding, I really didn't lose much weight. Maybe a few pounds at most. The difference in how my clothes fit, however, was remarkable. I ultimately went down two sizes and lost some excess fat. Keep in mind that it wasn't my goal to lose weight. I just wanted to ride to ride.

As another poster suggested, keep upping your mileage and speed. To really see significant body changes, you'll want to maintain a decent pace (one fast enough to get your heartrate up) for at least an hour.

Speaking of speed, are you spinning your pedals or mashing (pushing) them? If you don't know what I mean, so a search on the forum for "cadence" and read up on it.