Was Rachel Frederickson's big reveal the, er, biggest in Biggest Loser history? Personal trainers Jillian Michaels and Bob Harper have seen some incredible transformations over the years, but even they were flabbergasted by Frederickson's 155-pound weight loss on the show's season 15 finale on Tuesday, Feb. 4.

Neither star could hide their shock as the 24-year-old former high school swimmer walked out onstage for the first time since leaving the ranch last fall. Both looked completely stunned by her appearance, and Michaels could be seen mouthing "Oh my God" several times before the camera panned away.

As previously reported, Frederickson lost a record-breaking 59.62 percent of her total body weight, shrinking down from 260 pounds and a size 20 to 105 pounds and a size 0/2. As a result, she beat fellow finalists David Brown and Bobby Saleem for the $250,000 grand prize.

"It's crazy!" the 5-foot-4 former athlete told Us Weekly of the money. "But it's not about that. [My body] is absolutely the prize -- to have this energy and to feel alive means the world."

The last time viewers saw her, Frederickson was competing in -- and winning -- the Biggest Loser triathlon, a challenge she described to Us as "so fun." The triathlon "was a great way to round out my journey," she gushed. "I lost that athlete, I lost that inner confidence, that self-worth. Throughout the season I started to find that girl again, and at the triathlon, [she] was there.

She continued: "I embraced her, and I said, 'Run with her. Go take charge of your life. You've had this second chance, now use it. Don't ever let it go.'"

Watch Frederickson's big reveal above!