Does stair master make your butt bigger?

Not normally: Exercises that will work your buttocks include: lunges, squats, dumbbell squats, step ups, toe taps, plie, squats with kickbacks, hip extensions, one legged dead-lifts and the gamut of aerobic exercises (running, swimming, bicycling, kickboxing, walking). However, if you are looking to make your gluteals larger- would suggest you seek input of a trainer.