ABC News' Felicia Patinkin reports:

H&M made headlines last month when it featured plus-size model Jennie Runk in its new swimwear ad campaign.

In an industry where swimwear is traditionally modeled by willowy or waifish models, Runk's appearance in the retailer's general -not plus-size - swimsuit campaign sent a strong message.

Runk, 24, is 5 feet, 10 inches tall, and wears size 12 or 14.

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In an open letter this week posted on her Facebook page, and reprinted by the U.K.'s BBC News, Runk railed against the general obsession with size.

"People assume plus equates to fat, which in turn equates to ugly. This is completely absurd because many women who are considered plus-sized are actually in line with the American average," she wrote.

Runk, who was discovered when she was 13 years old, had a choice to lose weight and remain a size 4 or gain weight and kick-start her career as a plus-size model.

"I knew I was going to end up gaining weight anyway," she said in an interview with ABC News' Bianna Golodryga that aired today on "Good Morning America." "I was getting hips, I was growing into a woman, so I figured it was easier."

That kind of confidence has served her well in an industry where some retailers - most recently Abercrombie & Fitch - don't even offer clothing in her size.

Asked how she avoided negative thoughts about her body, Runk said: "I am the only one who can judge me. My opinion is the only one that matters when it comes to me."

Runk credits her success to the many other so-called plus-size models who blazed the trial before her, including Robyn Lawley, designer Ralph Lauren's first plus-size model.

"I met a lot of them and they taught me a lot of what I know now," Runk said.

Runk hopes she has helped start a conversation that may change attitudes about beauty and the so-called "mean girl" culture that sometimes comes along with it.

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"Our bodies are built to be naturally different sizes," she said. "To denote any of these body types negatively is only hurting all of us, because that's where you get girls of one body type slamming another to make themselves feel better."