Ann Edmonds's eyes well up as she stands in front of the mirror in a pop-up shop for plus-size women's clothes in Glasgow's Buchanan Galleries.

"Wow," the charismatic and vibrant 58-year-old keeps saying, as she looks at herself in a dress by plus-size brand Marisota - which has set up shop for just four days in the heart of Glasgow consumer-land.

This is a Cinderella moment, the kind of climax that television makeover shows build to, but here it has happened within minutes of Edmonds entering the shop. "I was totally emotional when I put this dress on - and I'm still actually shaking inside," says the amateur actor.

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"I'm not going to give you a big sob story, but my life has been pretty crap for 12 years. I lost my father, and my mother's in a nursing home. I retired from teaching 12 years ago because of my own health problems, and this is like wow, this is really good. This has really lifted me."

When was the last time she put on a dress and felt like that? "So long ago I can't remember," she says.

Edmonds is the kind of woman we don't often come across in fashion magazines. There are many more plus-size women like her in the world than there are size zeroes, but we simply don't see them. A peculiar thing has happened: at the same time as we have seen our society gain in weight, with a rise in national obesity, the ­representations we have seen in the media and in fashion of female bodies have got even thinner.

Twenty years ago the average fashion model weighed 8% less than the average woman. Today she weighs 23% less. As a nation we are getting bigger, but that is barely represented in the media.

Nor do we see it on the high street, where the shop windows for the most part are still full of size 10 (or below) mannequins, nor on the hangers in those stores where all too often the on-trend fashion seems to stop short of our national average of size 16.

All this can only mean more body-image anxiety. But while parts of the fashion industry seem relatively resistant to altering this trajectory - last week there was controversy following Vogue editor Alexandra Shulman's confession in an interview with Lily Allen that she was "bored" by the issue of why models are so thin - there are retail brands attempting to revolutionise the market for plus-size women, and in doing so make them feel good about themselves. One of these is online plus-size brand Marisota, and it is marketing its approach through a rather scientific sounding concept called shapeology.

According to one of its designers, television stylist Mark Heyes, it actually turns out to be "a combination of technology, clever cut and common sense". This weekend it has been presenting this shapeology in a pop-up shop in Buchanan Galleries, Glasgow. And it seems to work. Women who haven't been able to find clothes to flatter their bodies are literally crying in the pop-up store at finally putting on a dress they feel good in.

What Marisota is doing is not just providing fashionable clothes for bigger sizes, but focusing on what we know flatters particular shapes - for instance, distracting from tummies with illusion panels, or ruched sections of fabric.

Marisota research found there were women out there who had missed graduations, family weddings and other major events simply because they did not have the clothes they could feel confident in.

"They don't know what to wear," says Glasgow-born Heyes, "and therefore don't want to be seen out in public. It's horrible. I'm aware that I work in fashion and it's frivolous, but it does actually change how people feel about themselves. It really does."

Heyes, the fashion stylist for Good Morning, says he now only dresses one size 10 model for the show - the rest are all bigger. "It took me a while to get to this stage," he says. "There were a lot of male bosses who didn't like it. They like to keep their models skinny, so it took me a long time to break that mould, but I did it. "

The women walking into Marisota are, not surprisingly, concerned about the size and shape of their own bodies. Almost all say they feel they need to lose weight. Susan Milne, a nurse specialist, says: "I know I'm too heavy. I try to do something about it. Of course, I do big-style need to do something about it. But actually it's not happening - and I still need to buy clothes."

But they also complain they feel confronted by high street stores and a fashion industry that almost does not seem to acknowledge their existence. Mary Sinclair, on a shopping trip with her sisters, notes: "Clothes these days are all geared up for the youngsters, skinny, tight. You know you get to a certain age and you start putting on weight around here and on your bottom and your back."

We have a culture that forces fat women be less visible - to hide. Even Ann Edmonds, ebullient as she is, says her weight has kept her off the stage in recent years. "I used to dance and sing on stage with amateur companies in Glasgow at the King's Theatre. But because I've put on so much weight I do backstage work. I'd love to be on stage again but I won't put myself on stage at this size."

Marisota is not a lone brand or voice. In fact, there is a growing movement towards embracing bigger sizes, even if it seems as yet to be taking place mainly on the fringes of the fashion world. In November, Debenhams introduced its first size 16 mannequins. Last year was considered the year of the plus-size model, in which Robyn Lawley became the face of Ralph Lauren and H&M used Jennie Runk for its swimwear campaign.

Often it seems that fashion-industry figures who want to decry attempts to get bigger women into the media, or to get larger mannequins onto shopfloors, do so by saying that actually as a nation we need to shape up.

Of course, on one level they are right: this is a health issue. It has been predicted that, by 2030, 40% of us will be obese. But seeing thinner models hasn't succeeded in making us thinner; nor have clothes with sleeves and waists and bustlines too many women don't fit into. All it has done is make us more anxious.

Meanwhile, one question is what is it that actually makes women, particularly bigger women, feel better about their bodies? The answer is partly more plus-size women in the media. A study from Durham University showed that seeing plus-size models photographed in aspirational settings helped reverse preferences towards slimness.

But more tangible again is what companies like Marisota are doing: enabling women to feel great by providing them with clothes that make them look good. You can see it there in the "wow" moment for Ann Edmonds, who just a few minutes ago thought she couldn't wear a dress, now standing in front of a mirror and loving the one she's in.