Kate Middleton took part in a friendly volleyball game for a charity athlete workshop. (Danny Martindale/Getty Images)

The "Fit Mom," whose photo of herself posing in a bikini with her three young sons caused an uproar this week, told ABC News that she applauds Kate Middleton for shedding her baby weight.

Middleton's physique drew attention after she inadvertently flashed her stomach during an impromptu game of volleyball at a charity sporting event Friday, revealing a flat stomach not visibly impacted by pregnancy.

Maria Kang became known as the "fit mom" after a photo of her posing in a bikini with her three sons under the phrase "What's your excuse?" went viral earlier this week.

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She said she hoped Middleton's appearance could help show others that by staying in shape during pregnancy and being healthy, it's possible to lose pregnancy weight quickly.

"I thought, 'Good for her,' and I'm in support for any woman who does well," Kang told ABC News. "I think it's important to applaud women who are doing well."

Back in July, Kate Middleton caused a stir when she appeared with newborn Prince George and made no effort to hide her post-pregnancy "mummy-tummy." But only three months later during the volleyball game, her stomach showed no hint of a post-pregnancy "tummy."

Kang, who used to be a personal trainer, said she thought it was important for pregnant women not to overeat during their pregnancy, which can make the post-pregnancy weight loss more difficult.

"I think women should not be eating for two first of all," said Kang. "You're eating for a little thing that's growing inside your belly. You should only eat an additional 300-500 calories a day. A lot of women are overeating."

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Middleton's first solo public appearance after giving birth came only a few days after Kang's photo went viral with people both praising Kang for her devotion to fitness and calling her the "fit mom" and others lambasting her for "fat-shaming" other new moms.

Kang said she was overwhelmed by the attention.

"I'm really surprised, I knew it was going to be a powerful image," said Kang. "I'm just a regular mom, I have a lot of priorities. I'm not Kate, I'm not a princess. It's not my job to look beautiful."

However, Kang said she strongly believed that with obesity rates remaining high, people should not be allowed "an excuse" for being out of shape.

"We have a healthcare crisis. We have childhood obesity," said Kang of the difficulty in dealing with obesity in the U.S. "What is the excuse we're making, and how do we solve this problem?"

However, new moms have become inundated with photos of Hollywood starlets immediately getting down to the pre-pregnancy weight. It's a microscope that worries, says ABC News senior medical contributor Dr. Jennifer Ashton.

"I think the pressures to immediately bounce back after birth are out of control today," Ashton said after Middleton gave birth in July. "After delivery, it takes a while for that muscle to contract, and that process takes a couple weeks."