In an interview with Into the Gloss the fitness guru revealed when the pair first met the now 40-year-old had a few "problem areas."
"She had 35 extra pounds on her. Her butt was long and she had outer thigh problems," she said. "Gwyneth is lucky because she's really tall, so she can hide it really well in clothes."
The extra weight was, of course, because the star had just had son Moses. So the trainer whipped the mom-of-two into superhero shape just in time for the first Iron Man flick to begin filming.
And who's bum did Anderson use as inspiration? No surprise here: Gisele Bündchen had the ideal booty shape.
"Before Gisele was even a household name, I cut her butt out of a Victoria's Secret catalogue, and I was like, 'I think every woman wants that butt,'" Anderson said. "The only piece [of the body] that I took from an actual person was Gisele's butt. I laugh about it now, but everybody still wants that butt. She's pretty timeless."
Now, seven years later Gwyneth does have a bum to be proud off. So proud, in fact, that she flaunted her perky derriere in a see-through dress at the premiere of the Marvel film's third installment this summer.
And we wouldn't expect anything less from the World's Most Beautiful Woman. No butts about it.