Why is my butt getting smaller?

I have been losing weight but I don't want my butt to get smaller! I did some squats and lunges the other day and my butt seemed to have gotten smaller:(. I want a body like Rihanna. I also have been doing the special k diet. What can I do at the gym to make it bigger. I wasn't fat to begin with I just wanted to tone up. Should I eat more and then workout more.
Why is my butt getting smaller?
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