...make your butt bigger.
You sit up and walk to your mirror, the weight of your newfound girth tilting you forward. You stand in front of it and turn to the left, while still looking at your reflection. You grip the amulet and erotically whisper "I wish my ass would grow."
After a second, you feel a tingling in your thighs and butt cheeks. You turn straight towards the mirror and smile as you watch your thighs grow outward, watching as they slowly and sexy inflate to a nice curve. You watch as the inflation slows, and eventually stop at a size that would get you stuck in a normal door. You run your hands along your new shape, but turn to the side again to watch your rear end grow.
Before this, you had a normal butt. But now you watch as it slowly inflates, reveling in the sensitivity of the new flesh. You place and hand on your right cheek and slowly rub as you inflate, using your left hand to slowly stroke your shaft. Your buttcheeks are now the size of basketballs, if basketballs were smooth and soft that is.
You continue to feel yourself as you grow for the next few minutes, until the growing eventually stops. You look back at the mirror and gasp. Your butt is now as big as a love seat, being only held a few inches over the ground. You fall backwards and smile at your reflection. Here you are, with a dick as big as your leg, testicles the size of beach balls, and an ass the size of a loveseat.
You stand up and try to pull on some leggings you finessed from your sisters dresser. You struggled for a minute, but eventually tore them. After throwing the ripped pants in the garbage, you sit on your bed. It's starting to get late, but your mind is full of possibilities. After lying on your stomach, masturbating to the new weight of your ass on top of you, you climax just as you think of what to do next.