When Lizzie Miller steps out on the catwalk on Wednesday for New York Fashion Week's first ever show dedicated solely to plus-size clothes, it will mark more than just a milestone for American women tired of the skinny waif aesthetic.

It will also cap the rise of one of the fashion world's unlikeliest new superstars: a bubbly Californian blonde who once attended Weight Watchers, was teased as a teenager for being fat and who shot to fame on the back of a single photograph that unexpectedly showed off her tummy.

Yet now Miller, 21, is about to become a big name in the fashion world's New York showcase, and she will be a symbol of a growing acceptance of female curves. "This is a huge step for the fashion industry. It is going to be a beautiful event. It is going to prove to everyone in the fashion world that it is beautiful to have plus-size women wearing these clothes," Miller told the Observer in New York last week, fresh from having a fitting for the clothes she will wear on the runway.

The news has already generated its fair share of approving headlines and some fashion writers are eager to see the show, which will feature numerous plus-size brands from America and Europe and is sponsored by OneStopPlus. "Personally I can't wait to see the photos! It feels this year like there's a plus-size revolution going on in the fashion world, don't you think?" wrote blogger Lindsay Ferrier.

In many ways that should not be a surprise. After all, some 62% of American women fall into the plus-size clothing category. At the same time there appears to be a cultural shift away from the "heroin chic" look that began in the mid-1990s and came to be symbolised by Kate Moss and her ultra-slim frame. Now hit television shows like Mad Men have harked back to the more curvaceous past and made stars of its notably curvy actresses, especially Christina Hendricks.

There has also been a backlash against some of the more notorious uses of air-brushing. Fashion firm Ralph Lauren was heavily criticised for altering a picture of one of its models so much that the woman's head ended up being thicker than her waist. French Elle brought out an edition that celebrated heavier women and German women's magazine Brigitte vowed only to use "real" women in its shoots.

Lizzie Miller, at 5ft 11in and with the enviable curves of a US size 12 (UK size 16), is happy to see a move back towards the era of Marilyn Monroe and Sophia Loren. "I am now really busy. I am working for clients that were once kind of on-the-fence about me before. But now they want me," she said.

It began with a picture that no one really knew would be important. It was August 2009, and the September issue of the US magazine Glamour ran an article about women feeling good about their bodies. Beside the text was a picture of Miller – taken from a fashion shoot that was almost 10 months old and had been shot for a different subject matter – in which a roll of Miller's stomach fat was clearly visible. Yet Miller's smiling face clearly sent the message: I don't care, I'm still beautiful and happy.

It struck a chord with Glamour's readers, who started commenting in their hundreds. The story went viral on the internet. Yet as it was spreading across the globe Miller herself had little idea that her life had just changed. She first heard that Glamour had used the picture when a friend texted her and she walked into a shop to buy a copy. "I thought: it is so small, no one will probably see that. Those were famous last words. It really was crazy what then started happening," she said.

Within a few days, the media requests started to flood in. Soon she was appearing on the Today Show and CNN. The picture of Miller had coincided with a moment in American popular culture that was waiting to happen: a desire to strike a blow against the prevailing fashion order that said women should aspire to be stick-thin. Now Miller believes it heralds the genuine acceptance of plus-size models into the fashion mainstream. "I don't think this will be like every other time when something happened and it was just a flash in the pan. I feel that it will be different. My picture was a ripple but it has been snowballing ever since. Now this (Fashion Week) show will be the next logical step," she said.

The pressures on women to aspire to impossible body shapes are something that Miller knows all about on a personal level. She has always been big; even as a young girl growing up San Jose near San Francisco in northern California.

By the time she was 11, living on cheeseburgers and pizza and with a natural tendency to put on weight, she had ballooned in size. "I was known as the funny, fat girl," she said. "I was not happy with that."

She decided to join Weight Watchers and eventually shed 60 pounds on the programme, learning to eat more healthily and seeing fast food as a treat, not a meal plan. But she could not change her basic genetics. She remained tall and big-boned and wide around the waist.

She was also teased at school. One boy in particular ridiculed her for wearing shorts or short skirts, saying her legs were fat and ugly. Miller found the experience so traumatic she changed the way she dressed and became convinced her body shape was unnatural and unappealing. "It was horrible. I was so insecure. It really scared me," she said. Eventually she realised that the problem was her perception of her own body, not that of other people.

With growing confidence, and an ever taller frame, she eventually went to a modelling search event in San Francisco. She was contacted by a few agents and a year later signed with the Wilhelmina agency, at the age of 13.

Miller has an openness and honesty that feels refreshing in a fashion industry that too often seems to thrive on neither. Nor is Miller's belief that things are changing an entirely universal one. The fashion industry's untouchable gods still seem to dictate an aesthetic that is far beyond the reach of ordinary people. Recently British designer Julien Macdonald slammed the idea of a plus-size model winning Britain's Next Top Model: "You can't have a plus-size girl winning – it makes it a joke."

But it is no joke to Miller. Ever since the Glamour photograph, she has regularly received emails from women and girls unsure about their weight or suffering from being teased. She tries to reply to most of them, encouraging them to accept their bodies, eat healthily and work on changing their own mindset rather than believing the insults of others. Recently she got an email from a girl in Quebec in Canada who was being teased by a sister and told she was fat and would never be loved. "She said that she had a body type that was like mine and now had my photograph on her computer and that she now felt good about herself," Miller said.

Miller replied to the girl and the two have since exchanged messages. "We have been emailing back and forth a little. I get to be her penpal now," she said.

It is the sort of caring attitude that goes a long way in life but perhaps not so far in the notoriously ruthless fashion industry. But Miller does not care. She has long ago forgiven the boy in her school whose teasing caused her so much stress. She even says she is glad that it happened, as it allows her to relate to the people who write to her. "I don't hate him. In a way I am glad. It means I really know how people feel when they tell me things. I can reply to them: I know exactly how you feel," she said.

And she has also had a definitive last word over her playground tormentor. "Now I just laugh. My hips and thighs are how I make my living," she said.