Kourtney Kardashian now weighs less than her sister Kim.

The US socialite - who gave birth to her first son Mason Dash Disick in December - has astonished her sister with her rapid size transformation.

Kim said: "My sister weighs less than I do, and she just had a baby - so that's kind of fascinating to me. She looks amazing!"

Discussing how she managed to lose the baby-weight, Kourtney - who celebrated her 31st birthday at a party in Las Vegas on Saturday (24.04.10) - said it was all down to breastfeeding.

She said: "Definitely breastfeeding helps. Everyone says that, but it's really true - and it's just being a busy mom."

Recently Kourtney - who is dating Scott Disick - revealed she was finally getting back to a comfortable state with her body after putting on 40lbs during her pregnancy.

She said: "I'm just starting to get comfortable being in a bikini again. I'm excited. Scott told me recently that he could tell my body was changing. He also said he could tell I'm noticing it because I'll do these stupid dances in the mirror."