However fans have complained that this is discouraging for curvier women - many of whom want to work out but feel embarrassed and annoyed that they can't purchase the clothes they require.
So when the brand posted an image to its Instagram feed this week, featuring a curvier woman in Lorna Jane clothing, the reaction was overwhelmingly positive.
Women rushed to comment their appreciation for the change.
"All women whether they have curves or not are real! Beautiful picture and glad to see that diversity is being represented more and more by your company," one wrote.
"This is a photo I actually stopped and looked at. Wonderful to know you support us who don't have time to spent 3hours at a gym each but do enjoy an active lifestyle body .. do feel more comfortable going to your stores now," another wrote.
"Fantastic to see a non stereotypical model. This image will help inspire and motivate women starting their fitness journey. Thank LJ for keeping it real," one added.
Recently, the brand asked customers to 'register your interest for sizes outside of the standard range' on their Facebook. Lorna Jane Clarkson also posted a statement on their website to indicate that they might be expanding their sizes.
"Our activewear is available in sizes from XXS to XL, which is what we've historically had a customer base to support and is in line with fashion industry standards," she wrote.
"But, as an individual with strong beliefs, I always feel as though we can do more.
"We would love nothing more than for Lorna Jane to lead the industry in sizes outside of common fashion industry standards and we need your help and support to get there."