Hands on hips, shoulders back, head raised proudly, Christina Lewis poses for photos on her style blog. Here she is in a body-con miniskirt, with faux fur cape, and deep, vampish red lipstick. Here, again, in a tight Topshop dress, black biker jacket and ferocious, feathered collar. Then, once more, in a PVC dress, a studded waistcoat, and heavy Jil Sander shoes that look capable of breaking a breeze block with a single swing. She is tough, strong, beautiful. As cool and cutting-edge as they come.

Lewis is also a US size 24 (UK 26), and her style blog, Musings of a Fatshionista, has been building a strong following in recent months. On her site she describes herself as "fat and fancy as it gets", before writing that, "when your options are seemingly limited, how do you stand out and be as fabulous as everyone else? By not being afraid to take risks".

A 24-year-old graphic designer from Takoma Park, Maryland, Lewis began developing her sense of style while at art school in Philadelphia. Since then, she tells me, "I'm never afraid to put myself out there with what I wear. I always tell people I don't dress to look slim, I dress to look amazing!"

Lewis's site is one of a wave of new style blogs by women who are defying their marginalisation by the fashion world. So, for instance, there's Young, Fat and Fabulous, run by Gabi Gregg, a 23-year-old teacher from Michigan, who says she started it because other sites "didn't know what was going on on the streets". Bloggers in the US are also behind Fatshionable and the Manfattan Project; there are plus-size bloggers in France, Australia, Germany and Poland.

In Britain, Diane Dennis, a 34-year-old fashion designer from Birmingham, runs Fat Girls Like Nice Clothes Too. I ask whether she paused before using the word "fat" in her blog title, and she says: "No, not really. I wanted to be direct, to stick my fingers up at fashion, and say 'Yes, I am fat. So what?'"

The sites vary, but most feature photographs of favourite outfits, reviews of plus-size fashion lines, as well as images of some of the women who inspire them, such as singer Beth Ditto, and model Kelli Jean Drinkwater. Most recently, many have featured Gabourey Sidibe, star of surprise hit movie Precious, who has been transforming perceptions of beauty and body image these past few weeks, striding the red carpet in dress after couture dress.

These sites represent the point where two internet trends collide. The first is the growth of a loose collection of blogs affectionately known as the "fat-o-sphere". On sites such as the highly popular Shapely Prose, writers and readers discuss issues surrounding body image and fat acceptance (the idea that people should accept their weight, give up dieting, and commit themselves to being healthy at their current size.)

At the same time, following the enormous success of The Sartorialist blog – where Scott Schuman posts photographs of stylish people he encounters in the streets – style blogs have become more varied, and more focused. Thirteen-year-old fashion writer Tavi Gevinson has caused a sensation, and Ari Seth Cohen's Advanced Style site has been capturing "New York's most stylish and creative older folks". Style blogs are democratising fashion, offering much more diverse images than we're used to.

Fat people are perhaps the least visible group of all in fashion terms, a status that plays out in two related ways. First is the paucity of clothes available to anyone over a certain size, a source of sadness for many fat style bloggers. "I think people underestimate how difficult it is to stay current, because the options are very limited", says Gregg.

It is also a source of inspiration. Lilli Hingee, 31, who works in publishing in Melbourne, began her site, Frocks and Frou Frou, partly because "the plus-size market in Australia is really, really dreadful", and partly as a result of seeing other women struggle with what was available. Hingee is a self-described "curvy girl" – neither fat nor thin – and on her site she documents items in larger sizes available from international fashion lines. In the last months of 2009, she posted a picture of herself in a different dress each day, for a project she called "frockapalooza".

The other way in which fat women – indeed, almost all women – are marginalised, is in terms of fashion imagery, with an insistence on extremely thin, young, and generally white women in magazines and on catwalks. In the past few months there have been small signs that attitudes are changing. Last year US Glamour magazine ran an image of plus-size model Lizzie Miller, sitting naked and unperturbed by her small, soft belly, which provoked a huge response. Glamour editor Cindi Leive, was then inspired to run another photoshoot with plus-size models – again, all posed naked. This month, high fashion magazine V ran its size issue, featuring a photoshoot by fashion designer Karl Lagerfeld, which pictured burlesque star Miss Dirty Martini in an array of bras, nipple tassels and suspenders.

Both Glamour and V have also run images of plus-size models in clothes, but it's interesting to note how many shoots depict them either half or fully naked. The images seem intent on fetishising big women, depicting them as intensely sexual beings, creatures of great appetites – not an unusual theme on the internet, where fat fetishist sites abound. Kate Harding, the writer behind Shapely Prose, points out that "they put them naked, and it's supposed to be this revolution, and in fact it's just like 'Great! You objectified someone a little bigger!'"

Compare these images to those available on the mother of all fat style blogs: Fatshionista. This began as an online forum in 2004, and has since gone from strength to strength. Described as "a heady mixture of social politics, fat-girl memoir, and popular culture", it now centres around a personal blog run by Lesley Kinzel, and still includes a forum where readers discuss "plus-size clothes swaps", or where to get a sports bra.

But perhaps the site's most mesmerizing feature is its Flickr gallery, where anyone US size 14 (UK 16) and over can post photographs of themselves in their favourite outfits. So you have a stunning woman in a red dress, a tattoo spreading rapturously across her chest. There's another tattooed lady in a peacock tunic, her bright yellow sandals echoing her bright yellow hair. There's a woman in a pink and green flowered mod dress with pink tights. There are women in satin cocktail dresses with lace gloves; cartoon print tunics with cowboy boots; hotpants with a jumper and beret; in swimsuits, bikinis, and tight, tight T-shirts. Thousands of women staring defiantly at the camera, daring someone to suggest they're not gorgeous.

Clicking through these photos, you get a sense of just how easy it might actually be to change our visual culture. Dodai Stewart, deputy editor of popular women's website Jezebel, has written of her first encounter with Fatshionista that "while scrolling through the images, it was startling, at first, to see picture after picture of 'fat', plus-size, or heavy women. Not because of their bodies, or their clothes – they look fantastic – but because I just wasn't used to it".

I ask feminist and psychoanalyst Susie Orbach, author of Bodies, whether she thinks these sites could shift our cultural outlook, expand our notions of beauty. "There's no doubt that they could," she says, before explaining the way we respond to images. "How do we get from a trend for straight-legged jeans to wide jeans? At first we find the new image foreign and horrible, and then it becomes very present, and we start to feel there's something wrong with us because we don't conform to it, and we want to catch up. It's the same with body image". Because we constantly see photographs of very thin women, many of them Photoshopped, we spend our lives trying to fit that impossible ideal – a relentless, often futile quest. "If we were seeing images, not just of fat women, but of all sizes, we wouldn't be continually having to catch up in terms of our own body size," says Orbach, "so we wouldn't be wanting to change it. We might feel that we were represented, and then we could focus on other aspects of our lives".

Some claim that the sites promote obesity, that by showing stylish, proud fat women, they may encourage others to put on weight. Orbach disagrees. "I think thinness is what promotes obesity, because it promotes dieting, and dieting is one of the biggest causes of disturbed eating." Her one concern is that some of the photographs could be considered objectifying, "I'd much rather see women doing something, rather than just staring at the camera", she says. Stewart offers another take on this. "Fashion can be great," she says, "but the women we see in magazines, and on catwalks, often seem like a kind of clone army. They all look the same, and they're reduced to the status of coat hangers: the ultimate objects. Then you look at women on sites like Fatshionista, and the photographs are as much about their individual personalities as they are about the clothes. You see the style and the person combined." In fashion terms then, these women appear the ultimate subjects.

Orbach feels the sites are "trying to transform aspects of visual culture, which will be very helpful to people. I wish they were in the mainstream". Others also hope – and suspect – that this might happen. Kinzel suggests "it's inevitable that not just fat fashion blogs, but all fashion blogging, will have an impact. It's hard for fashion magazines to compete when you have amazing free blogs like The Sartorialist and Advanced Style. Fashion has always existed as a place that people look to, aspire to, and admire from afar, but never feel part of. That's changing. The sudden use of more plus-size models in fashion magazines is absolutely influenced, however vaguely, by fashion blogs that show a diversity of people".

There are certainly signs that these sites are having an affect on individual women. Ragini Nag Rao, a student from Kolkata, India, first started posting pictures of herself on Fatshionista in 2008, after struggling with her body image. "For the first time in my life," she says, "I was seeing these people who were my size, and larger, and who all looked great, and I thought 'If they can do it, why can't I?'" Nag Rao recently started her own blog, Forays in Fatshion, and when I ask whether this has improved her self-confidence, she says, "Yes, radically. It's transformed me, really".

Many of these bloggers see what they're doing as a political statement, as well as an enjoyable pursuit. "Being fat and comfortable with yourself, and calling yourself fashionable, is in itself political," says Gregg. "It runs against what the mainstream considers possible". Nag Rao agrees. "Putting pictures of myself up on the internet is my small act of fat activism. When I upload my pictures, I always tag them with the words 'obesity epidemic' and '200lbs' because this is what the obesity epidemic looks like. It's not the huge, headless fatty that you see in the newspapers. This is it."