Cycling will not give you a bigger butt, but it may give you a more shapely one. Cycling works your legs and glutes, especially when you are climbing, but it is does not last long enough or provide enough resistance to build big muscles. Overall, cardio activity, such as cycling, will be better at burning fat surrounding your glute muscles than it will be at making your butt bigger.
Cycling is often seen as a form of high-intensity training, especially if you are taking an indoor cycling class or cycling outdoors on hilly terrain. The up-and-down, interval-like ride is the most efficient form of cardio in terms of burning fat and speeding up your metabolism. While your muscles have to work harder during the course of your ride, the difference you see will be in terms of fat loss rather than muscle gains.
Muscle builds when enough stress is placed on the muscle, such as with weightlifting. Lifting actually tears the muscle, and the growth occurs when the muscle is in repair and its fibers increase in size. Cycling does not place enough load on your glutes to cause hypertrophy, or muscle growth. In fact, too much cardio can actually hamper muscle gains by slowing recovery and burning up calories that your body needs to build muscle.
When cycling, climbing is the time when you will feel your butt kick into high gear. Your legs need to access their power to push you up the hill. Yet most climbs only last a matter of minutes, which is not near long enough to build big, bulky butt muscles. While climbing will make your legs and butt stronger, strength doesn't always come across in size.
Amount of Cardio
It is common to hear of rest days between strength-training sessions because the muscles have to recover. But recovering from high-intensity cardio like cycling is also necessary. Your glute muscles will be getting stronger and more shapely as you ride, but rest time will need to be incorporated to recover and prepare for your next ride. Take a day in-between vigorous rides to rest your muscles.