Just call it the incredible shrinking behind! KIM KARDASHIAN stunned friends and fans when she showed off a strikingly smaller posterior during an appearance in London on May 17 — leading to speculation that she may have had surgery or removed the "butt pads" she was rumored to wear.

But those close to the 31-year-old reality star say it isn't so. They insist she's shrunk her buns the old-fashioned way!

Kim has long maintained that her nether region is all natural. she even flashed X-Rays on "Keeping up with the Kardashians" to prove it. But the bootylicious beauty began packing on the pounds after filing for divorce from basketball star KRIS HUMPHRIES late last year, and sources say she was stunned when she realized how big she had become.

With a hectic schedule ahead of her — including three more seasons of her reality show to film, plus promoting the weight-loss system Quicktrim — Kim knew she had to take drastic measures. Insiders say she embarked on a take-no-prisoners weight loss and workout program and has already lost 20 pounds!

"Kim took a look in the mirror a few months back, and said, 'I look like a blimp and my rear end is massive! How can I promote a weight-loss regimen looking like this?'" said a source. "She immediately got to work, determined to look her best."

Pick up The ENQUIRER this week on newsstands for more details!