Credit: Denise Truscello/

Wedding slim down!

Kim Kardashian is planning on losing some weight before she says "I do" to Kris Humphries.

"Starting QuickTrim today," the reality star, 30, tweeted on Wednesday. "It's officially summer. I want to really get in fab shape!"

A few hours later, Kardashian told her followers she'd "just finished my second workout of the day!"

"I'm on a mission," the famously curvy businesswoman added. "Feels good to be focused and dedicated."

Despite the world -- and her fiance's -- obsession with her behind, Kim envies her sister Kourtney's tinier frame. "My sister weighs less than I do, and she just had a baby -- so that's kind of fascinating to me," Kim told Us Weekly in April 2010.

When Kourtney, 32, revealed that breastfeeding helped her to shed the pounds, Kim joked that she might have found her new fitness plan. "If that's all it takes, breastfeeding?" she quipped. "Then someone breastfeed off of me! I don't care!"