I worked as a physician (not anymore nowadays, got into medical systems design), and I can guarantee you that either people and health professinals overemphasize the importance of the weight as if it was a single "magic number" regarding health, and it is not.
Body fat percentage, cardiovascular endurance, muscle and joint flexibility, dietary habits and psychological stress level are much more important.
Besides that, two things you must consider:
- When one increases pedalling distances and speeds, fat is lost and muscle is gained. Muscle is much more dense, so even if you look lean, you do not lose so much weight, but you exchange a very risk-related tissue (fat) by a much more healty one (muscle). And even while at rest, muscle burns calories much more than the same amount of fat, so the very existence of more muscle implies a higher methabolic rate;
- When you commute by bike, you get REALLY HUNGRY. You must eat, because food is your gasoline. Actually, wanting to lose weight by eating less then necessary is dangerous for bike commuters, because you might get weak during a ride, which greatly increases discomfort and accident risk.
What I do is to eat just enough, take a lot of salad and vegetables when possible (helps to provide anti-oxydants for the body, much useful for us who breathe pollution, etc.), and try NEVER to ride "out of fuel".
EDIT: (from wikipedia)
Some common body fat percentages (not a recommendation!)
Minimal 10–13% 2–5%
Athletes 14–20% 6–13%
Fitness 21–24% 14–18%
Average 25–31% 18–24%
Obese 32%+ 25%+
"The bioelectrical impedance analysis (BIA) method is a more affordable but less accurate way to estimate body fat percentage. The general principle behind BIA: two conductors are attached to a person's body and a small electric current is sent through the body. The resistance between the conductors will provide a measure of body fat, since the resistance to electricity varies between adipose, muscular and skeletal tissue."
I am not a native english speaker, so some words might not be correct.
Hope it helps!