In the UK, 45% of women are a size 16 or above, yet they often feel poorly served by the fashion industry.
These curvy ladies who love their clothes are faced with frumpy, poorly fitting outfits and scaled up versions of size 10 garments, which rarely suit a plus-size lady.
Trinny and Susannah want to prove to retailers that bigger women can look just as fabulous in stylish clothes as slim women, and aim to design their own plus-size range.
Embarking on their campaign, the girls want to know what it feels like to shop as a larger lady, so they dress up in padded suits to scale their bodies up to a size 20.
Trinny says: "I think [wearing the padding is] going to draw attention to the issues so when we're trying on trousers - how difficult it is when your thighs are going to make the trousers pull? So we'll be able to address the issues of dressing."
To help them, they meet two women who love their plus-size: 38-year-old company director Geraldine, a size 22 and proud of it, and 30-year-old credit controller Vicky, who is size 20.
Geraldine says, "I love my face, I love my hair, I have a nice little belly which I rest my plate on, I love my boobs, I think they're fabulous - D/DD - excellent!
"Designers seem to think that because you're big you don't have curves, you are just a blob. We're not a blob, we have curves. We go in at the same place as a size 10 but we just start a bit bigger."
Vicky is a dedicated follower of fashion, but feels let down by what is available.
"I love my fashion magazines and watching all my fashion shows, things like this, and it just seems like it's a world that's excluded to me; I'm just on the outside looking in.
"You just feel you're so discriminated against. You see these gorgeous clothes on the high street and think, 'I'd really, really like that, but I could never go in this shop'. I wouldn't even attempt to go in just to have a look."
Trinny and Susannah get a taste of plus-size shopping and soon realise that shopping for a larger lady takes a huge amount of time and effort just to find things that may fit them, let alone whether they might suit them.
"You see all these beautiful clothes and you can't go anywhere near them," says Susannah.
"It's like they're ring-fenced by barbed wire."
The pair decide they can show retailers that plus-size clothes can be designed well.
"If we can't create better outfits than those that are out there already, it's like admitting to the retailers their clothes are good enough, and that's failing nearly half the women in Britain," Susannah says.
"What we're learning is what all the retailers and manufacturers know already - it is hard to dress larger women."
• Trinny And Susannah Undress The Nation screens on TV ONE at 9.30pm on Friday.
