The Kardashian sisters celebrated Kourtney's 31st birthday on Saturday at Wet Republic in Las Vegas. They stayed uncharacteristically covered up for the cameras... maybe those tabloid bikini covers pay more that way? Kim told Us Weekly that Kourtney, four months after giving birth, already weighs less than she does. Kim is 5'3" and Kourtney is 5'1." Kourtney attributed her speedy weight loss to breastfeeding--a strategy Khloe would like to try.