I think you have a misconception of what cycling will do for your weightloss. I don't view riding a bike as a necessary guarantee. Despite all of the fad diets etc. you basically need to burn more calories than you eat in a day. That doesn't mean for one day. That means for every day, each day of the year, each year of your life.
I am not a physician but if you are healthy I can think of no reason that your waist could get larger unless you are burning fewer calories than you are consuming.