Rihanna may be one of the biggest stars in the world, but the singer still does get insecure at times.

When speaking with E! News' Alicia Quarles during her MAC cosmetics promotion, Rihanna admitted that there's one part of her body she'd like to be bigger - her butt.

"I do pick on my body," Rihanna said. "It's a thing that women do. We walk in [front of] the mirror and we look at our butt. Is it getting bigger today? Ew. It's still flat."

After Rihanna's comment, Quarles joked that her bottom is too big.

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"Hand it over," Rihanna joked. "You see that is what I call selfish."

Rihanna has been busy promoting her new role as MAC's first-ever celebrity "creative partner." The Pour It Up singer is the face of Viva Glam, a lipstick and lipglass with all proceeds going to the company's AIDS Fund.

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The 25-year-old also visited Good Morning America on Wednesday to speak about the project and even sported her MAC lipstick.

"I want you to have fun," Rihanna said. "I love having fun but you have to be responsible and safe and protect yourself. You have to look out for yourself because no one else can do that for you. It's a real disease."

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