The reality TV star has resorted to a tough exercise routine and weight loss pills in a bid to lose 20lb.
After a workout with her sister Kourtney - who gave birth to daughter Penelope Scotland in July - Kardashian tweeted: "How is it that @KourtneyKardash & I have the same amount of weight to lose but she just had a baby! SMH help! LOL (sic)".
She added: "Since I started QuickTrim again a few weeks ago, I'm down 6 lbs!!! Yay best feeling. I was getting discouraged but now I'm so motivated!"
Kardashian has previously admitted that she is not "ever really comfortable" with her own body, and was recently quoted as saying that her "comfortable" relationship with rapper Kanye West has affected her body shape and left her feeling overweight.
She told Life&Style magazine: "I'm hoping to slim down a little bit, and I'm not afraid to say it. I think everyone goes up and down in whatever that comfortable love relationship phase is where you like to eat out, but now it's time to get it together again."
A source also suggested to the magazine that West is the biggest champion of Kardashian's weight loss plan and is keen for his girlfriend to have a body like Beyoncé, saying: "He suggested Kim drop 20 pounds, arguing that Beyoncé has a similar shape and she proved you can get that supermodel body using her lemon juice, cayenne pepper and maple syrup diet.
"He says he wants her to be the face of his clothing line... but to do that she has to fit into the clothes. It's really uncomfortable - her shape is what made her name."