Jessica Simpson insists she's happy with her curves, but pals say she's jealous of her younger sister Ashlee's super-slender figure.

"Jessica wants everyone to believe she's learned to embrace her shape, but the truth is she'd give the world to have a skinny body like Ashlee's," a friend told The Enquirer. "Although Jessica claims her new show has helped her overcome years of body image issues, she's more insecure than ever about her looks."

"She's exercising like crazy with an intensive boxing, jogging and weight-lifting regimen, and she's following a strict no-sugar, low-carb diet. Yet she still can't can't lose the extra weight."

"While promoting her new show, 5-foot-3 Jessica hinted at her self-image struggles, admitting: "I haven't always looked at my reflection and loved it. There's always something that I've wanted to fix because there's always somebody that looks better."

And that somebody is 25-year-old Ashlee, sources say. At 5-foot-6, she's still a lithe size 0 even after giving birth to son Bronx in November 2008. "Jessica loves her sister dearly and wishes she was blessed with the same fast metabolism that Ashlee has, who barely has to exercise to stay fit," the friend divulged.