It's not easy being the "biggest" Kardashian sister.

In the new issue of Us Weekly, out Wednesday, Khloe Kardashian admits red carpet events with older sibs Kim and Kourtney can take their toll on her self esteem.

"They are 5-foot and 5-foot-2, so I look massive next to them," Khloe, who is 5-foot-10, tells Us. "Everyone expects me to be 9 feet tall and weigh 200 pounds [when they meet me]."

26-year-old Khloe -- whose mom Kris Jenner even criticized her weight in a recent episode of Khloe and Lamar - has "struggled with her weight since she was 8 years old," a family friend tells Us. "She was always taller and developed much broader shoulders than her petite sisters."

And the E! reality star, happily married to basketball star Lamar Odom for over a year, says she first gained weight following Kris' divorce from her late father, famed OJ Simpson trial attorney Robert Kardashian Sr.

"My brother [Rob] and I were always heavy. They gave us food to keep us comforted. All we did was eat crap," she recalls.

Classmates and even her mom's friends noticed that Khloe's older sisters Kim, 30, and Kourtney, 32, stayed skinny despite the family turmoil. "I had older women tell me, 'You should really work out,'" she explains. "It really got to me."

Her initial response? A teenaged Khloe hit the gym, working out three times a day and eating tiny meals. She went from a size 12 to 0.

"I started to get a positive response from people when I was losing weight," she admits.

For more of Khloe's revealing, emotional sit-down with Us -- her scary wakeup call at age 15, her "embarrassing" breakdown on the show, what Kim and Kourtney have to say and more -- pick up the new Us Weekly, out now!