Kim Kardashian: Kourtney Weighs Less Than Me!

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Kourtney Kardashian's post-pregnancy diet has worked so well that she's skinnier than little sis Kim.

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

PHOTOS: See what the Kardashian sisters looked like as kids

"She looks amazing!" continued Kim, who wore a black Robert Rodriguez jumper, L'Wren jewels and skyhigh Proenza Schouler heels. (Kim is 5-foot-3, while Kourtney is just 5-foot-1.)

So how did Kourtney slim down after having Mason last December?

PHOTOS: See how Mason has grown

"Definitely breastfeeding helps," she told Us. "Everyone says that, but it's really true -- and it's just being a busy mom."

Khloe, 25, wishes she could drop weight as easily as her sister.

"If that's all it takes, breastfeeding?" she joked to Us. "Then someone breastfeed off of me! I don't care!"