Americans are getting bigger, and retailers—after years of ignoring plus-size shoppers—are starting to notice.

Chains including Nordstrom Inc. JWN -0.16% and Target Corp. TGT 0.35% are boosting their plus-size offerings and displaying the clothing next to standard sizes, breaking with a practice of segregating larger sizes in a separate department often hidden away at the back of the store. The chains are adding supersize mannequins, and some are even showcasing plus-size models on their websites alongside the usual waiflike figures.

Fashion has long cultivated a body image that was out of alignment with most Americans. But the discrepancy is growing, making it harder for retailers to ignore as they grapple with rising competition from Amazon.com Inc. and other online retailers that has cut into sales and led to the closure of hundreds of stores.

More than 70% of U.S. adults age 20 and older were overweight or obese as of 2014, according to government health statistics. That compares with 67% a decade earlier. The mean waist size for American women age 20 and older was 38 inches, according to the data, which equates to a size 16 for most brands. Regular sizes typically start at 2 and run through 12 with plus-sizes starting at 14.

Men are getting bigger, too. Target offers some Big & Tall clothing for them next to regular sizes in select stores. And Nordstrom is hoping to have a bigger selection of plus sizes for men next year. But, for the most part, retailers are testing these changes in their women’s departments.

Nordstrom is adding larger sizes from 100 brands, many of which previously had not made plus-size clothing, including Italian luxury brand Gucci. The extended sizes will be available online and in 30 stores, alongside the regular-size clothing for each brand, and displayed on mannequins ranging in size from 2 to 18.

Target by year-end will carry plus-size swimsuits, athletic gear and lingerie next to regular-size items in those categories in 300 stores, up from about 150 stores currently. It is also expanding its existing plus-size departments in those 300 stores. More than 1,000 locations will have mannequins spanning sizes 4 to 22.

Outdoor chain REI increased its plus-size offerings by 50% during the past year. Sixteen of its 153 stores now display larger sizes next to regular-size parkas and other gear.

“There weren’t many outdoor brands that made larger sizes,” said Michele Orr, REI’s general merchandise manager of apparel. “We had to convince them by explaining the business opportunity.”

The moves are a departure for an industry that plus-size shoppers say makes them feel like second-class citizens.

“Plus-size clothes are often at the back of the store, the departments aren’t well stocked and the experience is so uninviting,” said Amanda Gilliam, a college-admissions consultant in Somerset, N.J. “It’s very shortsighted of brands not to make plus-sizes. They are missing out on so many people who are prepared to spend money.”

Some brands have shied away from manufacturing larger sizes because of the expense and complication of getting the clothes to fit. In standard clothing, the length and width increase proportionally for each successive size, according to industry executives.

In plus-sizes, the width increases more than the length, requiring manufacturers to create new fit patterns, which is costly. Larger sizes also require additional fabric, which adds another layer of expense, though clothing producers often pass many of these costs on to consumers.

“There are costs associated with having extended sizes and lots of brands just aren’t interested,” said Emma Grede, who with Khloé Kardashian founded Good American, a denim and T-shirt brand that runs the size gamut from 00 to 24.

Nordstrom began carrying Good American in October and noticed that 16 and 18 were among the best-selling sizes. Most of the other denim brands Nordstrom sells didn’t make jeans in those sizes, so it asked them to expand their range, according to Tricia Smith, Nordstrom’s general-merchandise manager of women’s apparel. Soon, it had enlisted other brands, including Rag & Bone and Theory, which added sizes 14 and 16.

Nordstrom is adding sizes at the lower end too, down to 00. And like Target, it isn’t eliminating its plus-size departments, though that is something Ms. Smith said she would consider if she could get enough brands to produce larger-size clothes.

“There is still a lot of work to be done,” she said. For instance, the larger-size mannequins had to be custom made because the standard mannequin size is a 2.

Not all plus-size shoppers want their clothes integrated with regular sizes.

“I’d prefer to have a separate department, because it makes it easier to find what I’m looking for,” said Veronica Miranda, who lives in San Francisco and is studying early-childhood education. She said she would welcome any moves that added more stylish clothing in her size. “It’s really hard to find current styles,” she added.

With the pickings slim, some retailers have begun producing the clothes themselves. Target introduced Ava & Viv in 2015, an in-house plus-size brand whose spring lineup includes off-the-shoulder tops and floral dresses. Since then, it has increased its private-label plus-size offerings by 50% with the launch of lines that sell both large and regular sizes, according to a spokeswoman.

REI started working with outdoor brand Kühl to produce plus sizes about a year ago. The brand previously hadn’t made plus-size clothing, but added sizes up to XXXL. It also began buying plus-size performance apparel from Shebeest and other brands.

To lend authenticity to its marketing, the company reached out to plus-size influencers such as Jenny Bruso, who writes a blog. REI sponsors some of her blog posts, as well as group hikes that she hosts around Portland, Ore.

Ms. Bruso said she started hiking six years ago, but had trouble finding the appropriate wardrobe.

“The outdoor brands weren’t making plus-size clothes, or else they were matronly and unflattering,” Ms. Bruso said. “Or, they were only available online, which sends the message that they don’t want us shopping in their stores.”

Corrections & Amplifications
Khloé Kardashian co-founded Good American, a denim and T-shirt brand. An earlier version of this article misspelled her name as Kloé. (May 8, 2018)

Write to Suzanne Kapner at Suzanne.Kapner@wsj.com

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