The creator of the wildly popular BBG program and Sweat app talks to PEOPLE about Katherine Heigl, her fitness philosophy and more

Celebrities are pretty lucky: If they want to get in shape, they can hire just about any trainer they want to come right to their homes and get their muscles burning. Which is part of why people were shocked to hear that actress Katherine Heigl turned to an app — Kayla Itsines’ Sweat app, which runs her Bikini Body Guide program, or BBG — to get her post-baby body.

But the Australian personal trainer, 26, is leading a fitness empire thanks to the popularity of BBG among celebrities and the average person. For Itsines, the best part about a celebrity like Heigl using her program is knowing how far it’s spread across the globe.

“I think it’s nice that women from all over the world are doing it,” she told PEOPLE at the launch of Westfield’s Beauty & Balance program in New York City. “I don’t mind if it’s a celebrity or anyone in this room. I just like that women are getting on board. And the fact that the community has reached so far that celebrities are using it as well, I just think that’s cool.”

Katherine Heigl’s BBG Instagram post
| Credit: Katherine Heigl/Instagram

Heigl isn’t the only famous name who is a BBG fan: Itsines says that Girls star Allison Williams has also used BBG, and she reached out to tell Istines about her love for the 12-week-long program.

“I had a lady message me saying ‘Oh I do your programs and I’m a big fan’ and it was Allison Williams from Girls,” says Itsines, 26. “And now we’re mates. In New York, I visit her, and we talk all the time. I was messaging her the other day, ‘I’m too scared to watch your movie. It’s on the plane, I’m too scared to watch it.’ She was like ‘Just do it!’ It’s nice to have those sort of people in your life.”

Itsines and her partner Tobi Pearce published their first Bikini Body Guide as an e-book in January 2014, never guessing how huge it would become. Itsines says she was compelled to create the program after seeing a massive hole in the fitness market when it came to workouts designed specifically for women.

“[By excluding women], you’re losing half the market,” she says. “I just wanted to focus on my market, which was women.”

But its popularity, she says, isn’t just because the program is for females — or that each workout session is only 28 minutes long. The community that’s developed around BBG helps women connect with each other and feel empowered.

Kayla Itsines
| Credit: Courtesy Westfield

“We’ve brought women together from all over the world through social media,” she says. It’s particularly popular on Instagram, where women share side-by-side photos of themselves at the start, middle and end, and Itsines frequently re-shares them on her own page. She thinks that the stories behind the photos are what makes the program so compelling.

“It’s not even the progress photos, it’s the stories behind them,” she says. “I think that’s what grabs them. If you read the story, it sort of changes your mind. I think that’s what is so powerful about the progress photos. It’s not I relate to her body type, it’s like I can relate to her story.”

For Itsines, fitness is all about feeling confident in your skin no matter what you look like. As she says: “What’s the point of being the most aesthetically beautiful person in the world if you don’t like what you see?”

Her philosophy? “Focus on how you feel, and not how you look.”