There is a difference between muscle development and definition. The reduction of body fat will cause the calves and legs to look more defined. If you cycle alot, then you will lose body fat in your calves and thus they will be more defined. If calves are small, or the muscle underdeveloped, and you wish to create more bulk, than weight training is needed. You can also ride occasionally "under load" by standing on the bike and pushing a harder gear. This is equivolent to standing calve lifts in the gym.