To get a bigger booty you need to focus on building up your glute muscles. These muscles are the gluteus maximus, gluteus medius, and gluteus minimus. Here are a variety of exercises and fitness programs that will help you do this.
Any exercise that has you engaging the glute muscles will enhance your bottom. Functional exercises such as squats and lunges are basic exercises that work the whole leg as well as the core. After mastering the basic forms of these exercises try some variations such as walking lunges, doing your lunges forward and backward, or stepping up onto a bench or elevated step with one foot. As you get stronger you can hold weights or dumbbells to add challenge and build even more muscle.
In the gym look for machines that work the leg muscles such as the stairclimber or the elliptical.
Many of the exercises in yoga and pilates also work all the glute muscles at the same time. Incorporate the warrior poses and chair pose in your regular yoga practice. In pilates try the superman pose or bicycling your legs while laying on your side.
If you are more adventurous try hiking, rock-climbing or cross-country skiing as a way of building up your booty muscle. These sports force you to take strong, deep steps and lunges and are especially effective when going uphill.
If you want to start off easy and do not have time for classes the best thing you can do is just walk or jog. By taking a route that may include some stairs or a small hill you will be working the muscles that will give you a bigger booty!