A big butt requires much more than bodyweight squats and lunges. The gluteus maximus is the largest muscle in the body and also one of the strongest. The glutes will grow quickly if given enough reason to, and the best way to accomplish that is very heavy exercise. Perform the right exercises strictly, consistently, carefully and with maximum effort and you'll see fast progress.
Leg presses are a good alternative to squats for people with knee and back problems because it provides stability. Leg presses can be superior to squats for building big glutes because the angle of the leg press forces the glutes to work through a greater range of motion. To perform a leg press, sit at the leg press machine. Use a wider stance to utilize the glutes more. Lower the weight sled until just before your butt rolls away from the seat. Press the weights back up, being careful not to lock your knees at the top. Perform sets of eight to 12 reps to build the glutes.
Lunges aren't a very heavy exercise, but because they primarily work one leg at a time, they can be very effective. Target the gluteus maximus by lunging forward with a long step. To perform lunges, stand with your feet together, holding dumbbells at your sides. Step forward, kneeling down until your front thigh is parallel to the ground, but not allowing your rear knee to touch the ground. Press with your forward leg and either step forward with your rear leg, or step back with your forward leg. Do the same with your other leg. Perform sets of eight to 12 reps per leg.
Dead lifts engage just about every muscle in the body though the target muscles are your glutes. You can use heavy weights for this exercise thus stimulating your butt muscles cells to grow. To perform a deadlift, stand in front of a barbell on the ground with your feet a bit less than shoulder-width apart. Squat down to the barbell, gripping it with your hands shoulder-width apart. The inside of your forearms should be pressed against the outside of your knees. Look forward, keep a straight back and press through your heels to stand with the weight. Follow the same path to set the weight back down. Perform four to six sets of eight to 12 reps for maximum muscle growth.
Once you get strong, you'll find that it's very difficult to perform a lot of big exercises on the same day. You might not be able to perform leg press and deadlifts during the same workout, because each requires so much energy. Stiff-legged deadlifts isolate the glutes more than the traditional deadlift, so you'll use much less weight. This makes stiff-legged deadlifts excellent for warming up before or burning out after bigger lifts. To perform a stiff-legged deadlift, hold a barbell in front of you with your hands spaced slightly wider than your shoulders. Stick your buttocks out behind you as you lower the barbell toward your feet. Keep a slight bend in your knees throughout the exercise. Contract your glutes to stand back up. Perform sets of eight to 12.