To exercise your gluteus maxiums and gluteus medias you can use a machine specifically to do so. Lunges, good mornings, and stiff leg dead lifts will have a significant impact in the area, too.
Improperly performed squats will overdevelop the whole area and are sure fire ways to give you that over-size ass.
Take some time to study the anatomy, identify the muscle, and then walk yourself through various exercises until you find the ones that will be most germane to your goals.
Squats WILL give you that big ass, but, and that's fine, but, you should also be sure you work your abs when you're doing it, or you'll likely end up with an imbalance that's a prescription for back pain later.