Since January, the close friends, who also work together, have been spreading this message to an audience of millions on social media. It started when the pair were hanging out and decided to don similar outfits, even though Mercedes is a size 14 and Castellano is a two. They posted it on TikTok in late January, and it's since received 1.5 million views.
"We were playing around, we did it mainly for fun," Mercedes, 28, told TODAY Style. "We didn't think it was going to get this much recognition."
After that initial video, they took their efforts to the next level. Instead of wearing similar outfits, they got the exact same items — a crop top and T-shirt dress from Zara — and shared their equally fabulous looks. That February TikTok post has almost 35 million views.
By then, the bloggers had started a movement, which they're calling #StyleNotSize. They've since seen dozens of other friends with different body types sharing their matching looks online, defying ideas of what cuts, colors and patterns look good on whom.
The almost entirely positive responses have been the most rewarding part of their work.
"I see girls commenting (on social media), 'I wish I looked like you,'" Mercedes said of the mindset they're trying to reverse. "Everyone is on social media 24/7 right now, and it can brainwash you into feeling bad about your body."
Castellanos, 26, added: "For us, it was trying to get rid of negativity ... Everything on social media is, 'This is how you’re supposed to look.' ... We’re giving you a hand and telling you to be yourself."
The comments on their videos, mostly from teenage girls and young women, have been especially striking, the pair told TODAY.
"I have girls telling me I’m helping them feel more confident," Mercedes said. "I wish when I was younger that I had someone to look up to. Back when I was 16 in 2008, it was always just a struggle to be skinny. I’m glad things are changing now."
Castellanos wants the success of their videos to change how existing fashion companies determine their size selections.
"It's been tricky to find brands that sell sizes zero to 20," she said. "We hope they can see there are a lot of different shapes and sizes and provide them clothing, as well."
Even though the retailers they've used have been limited, they've learned a lot about dressing a range of body types. Solid colors are better than patterns, they said, and American Eagle's denim fit both of them the best. But the most important is feeling comfortable in whatever you're wearing, Mercedes said.
What's next for the fashion-forward duo?
They hope to start a clothing collection that isn't organized around size, unlike stores like H&M and Forever 21, which have separate sections for plus-size items, Mercedes said.
And of course, they want to continue to spread more body positivity, especially for "the younger generation," Castellanos said.
"I would love to see women just be themselves," she continued. "I want them to be like ‘Oh my God, I wore this crop top, and I look good.' I want to feel like nobody cares anymore."