Not long ago, Kim Kardashian was world-famous for her sexy curves, while her pregnant sister Kourtney was stuck in the role of second banana. But eight months have made a world of difference for the ultra-competitive Kardashian sisters. After working hard to lose her baby weight, Kourtney has her slim body back, reawakening Kim’s old insecurities. And now, the two sisters are more competitive than ever. “They are always looking at each other’s body and picking it apart,” says an insider, adding that in the past, Kourtney was always the “skinniest sister,” and now that she is once again, she’s making sure Kim knows. “She always makes fun of Kim’s ‘lumps,’ meaning her cellulite,” the insider reveals.

According to a Kardashian confidante, it’s a battle that’s been raging between the two women since childhood. With Khloé, 26, considerably younger and always a bit bigger than her two sisters, Kourtney and Kim engaged in fierce competition as teenagers over who had the hottest, slimmest body — and who got the most attention from boys. Of course, now it isn’t just dates the girls are fighting over — it’s money, fame and airtime. Although Kim used to be far and away the most famous Kardashian sister, she’s seen Kourtney get more attention over her new baby and her tumultuous relationship with her boyfriend, Scott Disick. And now that’s she’s lost the baby weight, Kourtney seems to be taking Kim’s place as the hottest star of Keeping Up With the Kardashians. “Kim isn’t about to give up her throne without a fight,” says another family friend. Dressing up, hitting clubs — and working hard to slim down — Kim is determined to take back the lead role in her famous family.