Sit down -- that's where it is. Seriously, don't know exactly what you mean -- your butt is too small? And if you do squats properly, with correct posture, you can do them without hurting your back. But you do have to know how to do them correctly.
Ride horses if you can, biking, hiking on an incline. Lay down and do side squats to take the pressure off your back! Hope that helps!
I do bridges, but I'm not sure the romantic music would get the proper reaction!
By the way, squats are just one way to work your glutes. I don't do them because I have lower back problems, but you don't need to lift anything to work your glutes. There are a million exercises you can for them just lying on the floor, and every gym should have a glute machine. Lunges do similar things as well, and don't require squatting as much. I do glute exercises on a bench. Really, there's a million of them. Go on YouTube and I bet you'll find a ton of them being demonstrated.