"This is not helpful to the girls that follow you," one said.
"Irresponsible thing to put on Instagram when so many young women will now be comparing themselves to an image you've deemed 'fat.' People should engage their brain before posting rubbish like this, think of the consequences," another chipped in.
Another wrote: "You have to realise you're a role model for at least a few hundred thousand young women who now have the impression that this is 'fat'."
"You are an example to little girls! Stop using the word fat! Especially when you appear toned and athletic," another said.
Others called Genie's comments "disgraceful", "terrible" and "moronic".
Her divisive photo caption also caused some of Genie's fans to reassure her about her body, telling her she was "perfect" just the way she is.
"You look stunning to me," one person said.
"There isn't an ounce of fat on that body," another declared.
Even some of her high-profile fans weighed in, with Big Bang Theory star Kaley Cuoco writing: "I'm gonna punch you."
Fellow tennis star Gaby Dabrowski asked: "What ru talking about?"
Jennifer Brady responded: "Better than both … ugh," adding a expressionless face emoji.
Many of Genie's followers defended her comments, saying "it's okay" to struggle with body image.
"Genie you look amazing and if you don't think so, that's okay. Many young women struggle with body image. Keep doing you girl," one stated.
"She's a human too people, stop with the negative remarks," another threw out.
Genie — who made her mark at the 2014 Wimbledon championships as the first Canadian player to reach the final of a Grand Slam singles tournament — responded to the uproar, telling a fan who claimed she'd made the post for "attention" she was actually "poking fun at myself".
"Not looking for attention, I'm actually making fun of myself. And showing it's okay not to be perfect! Because none of us are. I'm poking fun at myself bruhhh you should try it, it's fun.
"Not take life too seriously and laugh with other people. I'm wondering why this happens."
It's not the first time Genie has clapped back at her critics, recently setting people straight about the lifestyle she documents on social media.
Despite bursting onto the scene when she stormed to the Wimbledon singles final back in 2014, she's better known these days for her high-profile photo shoots — notably posing for the iconic Sports Illustrated Swimsuit edition.
Yet when asked about her work in front of the camera, Bouchard told The Telegraph: "I think that's a big misconception.
"But I do some here and there when it fits with my schedule.
"I enjoy dabbling in different industries, meeting successful people in other walks of life, not just in tennis.
"It's more of a creative area, it's new to me and interesting, so I do enjoy it. But it's work as well.
"That's one of the things people think about me. I'm not bothered by it, but I am putting the facts straight. I cannot even remember the last time I did a photo shoot.
"The most important thing to reiterate is that social media is what I choose to show everyone. It's not an exact posting of my whole life."
