Hi,
you are on the right track. As the buttocks are made up mostly of muscle, exercises can make them bigger and rounder. It is important that you do exercises that puts heavy resistance on your buttocks. When muscle has to struggle it is forced to grow bigger. You need a workout designed to make your butt bigger, not just firmer.
Diet is also important if you want to make your butt bigger. There are two reasons for this. You need good protein in order to rebuild your muscles (buttocks in this case) between workouts. Good protein can be found in meat, fish, eggs and milk. Don't overdo it but increase the amount of protein you eat a little.
The second reason why diet is important is that some foods will actually help you to add some fat to your butt without putting fat on the rest of your body. Fatty acids and omega 3 are naturally stored in the backside of woman and eating foods such as fatty fish, fish oil or nuts can help you to add some fat to your butt and thus making it bigger and rounder.
Good luck!