It was March 2013 when Kayla Itsines and her partner Tobi Pearce decided to turn her affinity for women’s fitness into a commercial venture.

Within five years the 26-year-old, who has 8.6 million followers on Instagram and a series of wildly popular workout guides, has become a force to be reckoned with. In 2017, Australian Financial Review named the Adelaide-based personal trainer on their ‘Young Rich List’, estimating her worth at $63 million, and just last month millions of Americans watched her do push-ups on a coffee table on Good Morning America.

The numbers paint an undeniable truth: Kayla Itsines is a powerhouse. She is admired and adored in such abundance that women can’t help but pull on their best athleisure wear and pass over their credit cards at the very sight of her.

A post shared by KAYLA ITSINES (@kayla_itsines) on Jan 21, 2018 at 1:24pm PST

When we hardly have a shortage of aspiring fitness influencers, how did Itsines make it all happen? Through knowing what women want, some clever marketing, tireless nights, and attractive salad bowls, sure, but it’s Itsines’ washboard abs – the ones we’ve seen shared across every crevice of the internet – that are borderline iconic. Largely, in having the body to which so many women aspire, and in unabashedly sharing that body, Itsines built her kingdom piece by piece as if each selfie were a brick.

But in a culture obsessed with weight, making your body the crux of your empire comes with an eventual dilemma: Women’s bodies are policed so strictly, so harshly, in this Instagram age that daring to not bend to fleeting trends is akin to not matching your sports bra with your Adidas sneakers.

LISTEN: Some women are feeling so body-shamed, they refuse to get pap smears. Post continues after audio.

And suddenly, the body that Kayla Itsines was once celebrated for is what she’s now getting shamed for.

In accepting herself, Kayla Itsines – all 1.65 toned, trim metres of her – is rebelling against what a personal trainer’s body supposedly “should” look like in 2018. You only need to read the peppering of critical comments that season her social media feed to see that.

In case you missed the memo, Instagram culture in 2018 stipulates the "in" body is to be as toned as ever, but now with a special addition: The Butt.

The Butt, as in the trend that made Gold Coast-based fitness influencer Tammy Hembrow - of "Booty Building Guide" royalty - rise to dizzying success. In a few short years, the 23-year-old has built a community that rivals Kayla's; Hembrow has 7.7 million Instagram followers, a burgeoning athleisure line, endorsement deals coming out of her sweat glands, and, if it need be said, glutes that need to be seen to be believed.

Hembrow's golden empire is, almost entirely, built on The Butt.

Savvy as ever, Hembrow doesn't just have one glute-based guide, but three - At-Home Booty eBook, Gym Based Booty and Gym Booty Program 2.0. YouTube videos of the mum-of-two squatting and pulsing have been viewed by the masses; her most popular video 'BIGGER BOOTY WORKOUT' has been watched a cool 8.6 million times.

As I said - The Butt is in. Hembrow was a clever cookie to pounce on that.

A post shared by ⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀Tammy ???? (@tammyhembrow) on Dec 20, 2017 at 12:30pm PST

And if you can't get a booty through sweat and tears? Never fear! There are plenty of options to stay "on trend" there, too. Both butt implants and non-surgical lifts have become the fastest growing procedures worldwide since 2015.

Speaking to Mamamia in 2017, New South Wales beautician and Sculpting Haus owner Aysha Learmonth explained that her $289 Brazilian butt lift treatments are so popular because "we all want to see those results coming in quick."

One of Learmonth's most loyal customers is former Bachelor and I'm A Celebrity contestant Keira Maguire, who snaps and shares each bum-plumping treatment with her followers.

The bullseye of what a "perfect body" looks like has swung, and the size of a woman's bum has suddenly been conflated with her wellbeing.

All this, according to those of Itsines' fans who perhaps have too much time on their hands, means that the personal trainer should follow suit and hop on that train to BootyVille pronto.

But Itsines is not boarding. You see, she just doesn't care for a big bum. She likes her body as it is. And there's nothing like a woman who accepts herself to agitate the internet's cesspit.

"This is my body - all day - everyday," Itsines wrote on Instagram last month.

"Some people may like, some may hate it, but I love my body... People ALWAYS say things like 'don’t you wish you had bigger boobs, don’t you wish you had a bigger bum, don’t you think you need to lift more or have more muscle' ... mmmm NOPE!"

Because Itsines dares to not buy into the "your bum must be thick" trend, because she doesn't care for big glutes or quads, her shape doesn't fit what Instagram's 'healthy women' look like anymore.

It's bullshit.

What's worse is that the people who are keen to let Itsines know this are, overwhelmingly, women. The same women who will swim in depths of confusion when the 'perfect body' pendulum swings back to something equally isolating, like thigh gaps.

Which it will. Of course it will.

It appears some have forgotten that Itsines can do endless chin ups, push ups and burpees and would probably run rings around all of our fitness levels combined.

A post shared by KAYLA ITSINES (@kayla_itsines) on Jan 13, 2018 at 4:34am PST

Let's be clear: Whether a woman looks like Tammy Hembrow or Kayla Itsines is not what matters; what a woman can do with her body is. The more emphasis we can place on a woman's actions over her aesthetic, the better.

This week, Itsines addressed the scrutiny once again.

"I don’t want to change my legs in any way," the fitness influencer wrote on Instagram on Monday.

"I don’t want a 'bigger bum', I just want to be fit, healthy and strong."

And I think we can all agree that's what is truly important.