Her personal trainer explains why: hard work.
"She is super fit and very dedicated," trainer Nadya Fairweather tells PEOPLE. "We train whatever the weather and she never cancels – she's pretty hardcore."
After some unflattering bikini pictures of the princess were published in 2009, Beatrice (Bea to her friends) contacted Fairweather after a recommendation from her boyfriend Dave Clark.
"She wanted to build up her stamina and just make exercise a part of her life," says the London-based trainer, who runs the fitness company U-Shape.
Dubbed by some London tabloids as "The Incredible Shrinking Princess," the 5'4" Beatrice, daughter of Prince Andrew and Sarah Ferguson, became the first member of the royal family to complete the London Marathon – as part of a human chain caterpillar no-less – in 2010. Two years later, she went on to climb Mont Blanc.
"I might have gotten the ball rolling, but what she has achieved is down to her own determination and hard work," says Fairweather, who has witnessed the 25-year-old's impressive but gradual transformation over the past five years.
"If she puts her mind to something she will do it."
After some intense initial training, Beatrice now is down to meeting her trainer once a week.
"She loves boxing and the TRX where you use your own body weight as a workout which is pretty hard," says Fairweather. "Plus, she probably does between three to five sessions a week at home, depending on how busy she is, and she does lots of reading on nutrition - she's really interested in food and eating healthy."
On whether Beatrice struggles with the pressure of being a young royal, always in the public eye, Fairweather says, "She never talks about it, but it must be hard especially being so young. She is so positive about everything, it's amazing."