Diet is important in building muscle. Your ass is a muscle. If you want to get a booty and you can't get one, or some thick thighs, you have to change what you eat to contribute to building muscle when you work out.
Proper diet and exercise can contribute to a proper booty/thighs etc... too many women think they can get a proper ass just by eating. Some people are lucky with that, but if you want a tiny waist you have to take what you eat and how you work out seriously and hit some squats.