Although cycling is primarily a form of cardio exercise, it also works your glutes, quadriceps and calves. Depending on how hard you work and on your body's natural composition, riding a bike can be strenuous enough to act as strength training for your lower body, building the muscles in your legs and backside. If you gain muscle easily, you might need to adjust your workouts and stretch regularly to avoid bulking up.
The muscles you use in exercise are not all alike. You have three types of muscle fibers -- you use mostly slow-twitch fibers in endurance exercise, and for strength training you use two varieties of fast-twitch fibers. When you perform aerobic exercise or lift light weights, your slow-twitch fibers strengthen and their function improves. However, they don't gain much mass. When you lift heavy weights or overload your muscles in other ways, you train your fast-twitch fibers, which then increase in size. This growth, called hypertrophy, is what causes you to bulk up.
Because cycling is an endurance activity, you are most likely to strengthen your slow-twitch fibers with it. This means you can probably cycle regularly without seeing much hypertrophy. However, if you naturally have more fast-twitch fibers than slow-twitch, your body recruits them more easily and you might gain muscle mass more quickly. Your genetics determine your muscular composition -- if you are a mesomorph, a person with many fast-twitch muscle fibers and not much body fat, the relatively minor strengthening effect of cycling might be enough to increase your muscle mass.
If you find that cycling is making your thighs or calves too muscular for your taste, adjust your workouts to stop overloading the muscles. You only gain muscle mass when you fatigue the muscles, and you can get an aerobic cycling workout without fatiguing your legs. If you are using a stationary bike, set the resistance low enough that you can pedal for your whole workout without exhausting your lower body. If you cycle outdoors, choose a flat path to avoid working your calves and quads too hard.
Even if cycling doesn't increase your muscle size dramatically, it might cause your quads and calf muscles to shorten and bunch up if you don't offset it with stretching. Stretch your quads by kneeling on the floor and sitting back on your heels, or, if you have more flexibility, lying back over your feet. Stretch your calves by placing one foot in front of the other and bending your front knee, then sliding your back foot further back until you feel a stretch. If you have knee problems, consult a doctor before performing these stretches.