The founder of the "Healthy Is the New Skinny" movement is shares why this is an issue—and what we should do about it.

Thanks to the body-positive movement, more women are embracing their shapes and shunning antiquated ideas that they should look like the size 0 models they were exposed to while growing up. Brands like Aerie have helped the cause by featuring a more diverse range of models as well as vowing not to retouch them. And models like Ashley Graham and Iskra Lawrence are changing society's standards of beauty by landing covers of Sports Illustrated as well as major beauty contracts. Finally, women are being encouraged to celebrate their curves, rather than being told they're something to be ashamed of.

But according to Katie Willcox, the founder of the Healthy Is the New Skinny movement, there's a whole group of women being left behind: Women who don't fit the stereotypical label of "skinny" but wouldn't consider themselves "curvy" either. These women who fall somewhere in the middle aren't seeing their body types represented in media-and more importantly, conversations about body image, self-acceptance, and self-love haven't included them either, she says. (Related: Katie Willcox On How She Made Space for Herself In the Modeling World)

Part of this stems from the fact that models have been encouraged to lose or gain weight so they can fit into a "straight" or "curvy" category-knowing they'd never find work as say, a size 6, 8, or 10. "So many fashion brands are now expanding to include plus sizes, but they still aren't changing the models they use for their 'straight sized' or 'sample sized' clothing," Willcox tells Shape.

Today, curvier women have such a strong and powerful voice on social media, and they're appearing in ad campaigns because of it, says Willcox. These women are the go-to icons of body diversity that we like to point to and pat ourselves on the back for accepting-but no one really cares to hear about the millions of American women that fall in the middle, she says. (Ahem... Is the Body Positive Movement All Talk?)

"If a brand uses a size 2 or 4 model to advertise their plus sizes, people are going to immediately speak up and say, 'Hey, this doesn't represent us the right way,'" she says. "Considering that could make national headlines the next day, it forces brands to make sure they're actually using fuller-sized models to advertise those clothes. But because people who are in the middle (between a size 6 and 10) don't have that kind of power, they are still seeing ads with models that are a size 0 or 2 representing clothing that's catered to them."

And, from a business standpoint, there's little incentive for brands to change that:"They're just doing what they've always been doing, and it's been selling for decades," she says. "They aren't hearing enough voices stand up." (Related: These Women Are Embracing Their Stature In the "More Than My Height" Movement)

Part of the reason why there hasn't been a push to include "middle-size" models is that almost all of the conversations about size diversity, body image, or self-love out on social media are geared toward more full-sized women, she says. "That type of messaging shouldn't just be geared towards one size," says Willcox. "We need to even the playing field and make sure we include everyone, even those who fall in the middle."

The solution to all this seems simple: Brands need to add medium-size women into the media mix and stop limiting their messaging to straight-size or plus-size images only.

But Willcox also feels that as a society, we should look beyond the media, rather than letting it continue to dictate how we see our bodies. "It's almost like an abusive relationship," she says. "We're never getting what we really need from them, but we keep going to them saying 'please love me,' 'please tell me I can love myself.'" In reality, self-love is what gives you the ability to filter out these negative messages, she says. (Related: One Woman Proves Body-Positive Advertising Isn't Always What It Seems)

At some point, you have to disconnect from that to truly accept yourself, says Willcox. "No matter what the media portrays, you have the power to consciously choose how you feel about yourself, about who you are as a person, and what you contribute to society," she says.

In an ideal world, there would be no right or wrong way to have a body, all sizes would be created equal, and representation would be fair and equal. While we aren't quite there yet, it's dialogues like these-and people like Willcox starting them-that will continue to push the movement forward. (And this is why we've completely changed the way we talk about women's bodies.)