The Kardashian girls (from L to R: Khloe, Kourtney and Kim) posed for pictures Saturday (Apr 24) as the ladies arrived for Kourtney’s birthday celebration at the Wet Republic pool at the MGM Grand Hotel/Casino in Las Vegas. Kourtney Kardashian turned 31 last Sunday (Apr 18).

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 UsMagazine.com at Wet Republic in Las Vegas Saturday. “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? “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!”