Kim Kardashian: Kourtney is Disappearing

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Kourtney Kardashian has lost so much weight since giving birth to adorable little Mason, she's practically disappearing!

Kourtney was in Las Vegas celebrating her birthday with her sisters Kim Kardashian and Khloe, and according to Kim, Kourtney weighs less than her now!

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"My sister weighs less than I do, and she just had a baby -- so that's kind of fascinating to me!" Kim, 29, told Us Weekly at Wet Republic in Las Vegas Saturday, where she celebrated Kourtney's 31st birthday with Khloe.

"She looks amazing!" continued Kim. And we couldn't agree more! So how did Kourtney shed all that weight? "Definitely breastfeeding helps," she told Us Weekly. "Everyone says that, but it's really true -- and it's just being a busy mom."

Whatever she's doing is working like a charm, Kourtney looks absolutely amazing!