You'd never think it, but the petite 37-year-old says she piled on the pounds while pregnant with daughters Olive in 2007 and Elula in 2010.
"I gained 65 pounds (29.5kg) with my first baby and 70 (32kg) with my second," she told America's Fitness magazine.
"I had severe morning sickness both times, so I mostly ate supersize bowls of white pasta with loads of butter and cheese because that was the only thing that took away the nausea."
Constantly in the spotlight due to a successful acting career and her high-profile marriage, Isla admitted that she definitely felt the pressure to slim down for her career.
"I had to shoot Confessions of a Shopaholic about three months after I gave birth to Olive, so the producer, Jerry Bruckheimer, hired a trainer to work with me three times a week," Isla revealed.
"That was both inspirational and terrifying because I didn't want to let the trainer down. After I had my second daughter, I slowly lost the weight by breast-feeding, hiking [LA's] Runyon Canyon and working with a trainer every couple of weeks."
The Great Gatsby star says she now practices yoga five days a week to maintain both her body and mind.
"When you're really focused on your breathing, you're able to clear your head, and all the worries of everyday life go out the window," she said. "I love that feeling of being connected with my own body … If I'm stuck in traffic or have a bad moment in my day, I go back to my yoga practice and I breathe."
The best thing about yoga? According to Isla, it's all about the butt: "Every woman likes her tush in jeans when she's worked on it a little more!"
When talk turned to the favourite part of her body, the redhead went where no woman normally does; her stomach. "Even though it will never be flat again, it's still my favourite because it reminds me of my greatest achievement: my babies," she gushed.
"For me, so much about life is acceptance. You can look in the mirror and find a million things wrong with yourself. Or you can look in the mirror and think, 'I feel good, I have my health, and I'm so blessed'. That's the way I choose to look at it. I don't need to be perfect. I'm doing just fine."
We think Isla always looks awesome. Check out some other "yummy mummies" in the gallery below.
Yummy mummies: slim celeb mums