As soon as Heidi Zak studied the numbers, she knew it was time to sell a bigger bra.

Zak, founder of the online start-up Third Love, has spent three years building her line. The company now offers 59 sizes, and it is testing 15 more. But demand - especially for larger sizes - is only rising.

Consider, she says: More than 500,000 women remain on the company's waiting list for bras in sizes like 44G and 46K.

"As soon as you look at the data, it's clear: It's a market that's so underserved," Zak said.

Zak is among online retailers who are culling customer complaints, preferences and measurements and arriving at the same conclusion: American women, who on average wear about a size 16, need bigger sizes.

Yet plus-size apparel makes up just a sliver of the clothing on the market. Internet start-ups, armed with reams of data and often more nimble than traditional retailers, are filling that gap.

Instead of creating "plus-size collections," they are more often creating the same dress for every size - say, 0 to 36. They also are bucking long-held industry notions of what larger women should - and shouldn't - wear, seizing on an opportunity that mainstream retailers have long ignored.

They're clearly on to something: Sales of plus-size apparel have been on the upswing. They grew 6 percent, to $21.4 billion, last year, outpacing the 2 percent growth in the overall women's clothing market, according to research firm NPD Group.

Stitch Fix, an online styling service, recently began offering up to size 24W across 90 brands. EShakti, which offers customizable clothing in sizes 0 to 36W, has been steadily increasing its range with promising results: Sizes 14 and up account for 52 percent of the company's sales.

And at online retailer ModCloth, nearly three-quarters of clothing is available in every size from XXS to 4X.

"A lot of brands think very narrowly about who their customer is," said Matthew Kaness, ModCloth's chief executive. "There's this belief that plus-size customers simply don't spend as much - but that is only because of a lack of choice."

A company survey, he says, found that 80 percent of plus-size women would spend more on clothing if items were offered in their size. His sales indicate that, too: Plus-size shoppers buy 20 percent more frequently than other customers.

"Women and men have been getting heavier, but manufacturers haven't kept up," said Marc Cohen, director of retail studies at Columbia Business School. "Most retailers don't even go there - or if they do, they're very inconsistent about their approach."

Retailers have neglected those women for years, an oversight that many in the industry say comes down to discrimination. Designer Prabal Gurung, whose celebrity clients include Oprah Winfrey and Melissa McCarthy, says he has always offered up to a size 22, but most retailers "typically don't buy beyond a 14."

"Progress in the industry has still been slow," said Gurung, who has partnered with Lane Bryant to create a line for women sizes 10 to 28. "For the longest time, they have been pushed aside."

And, designers and stylists say, the industry's sense of "normal" has long been skewed by runway shows and high-fashion magazines filled with rail-thin women.

"The fashion industry still considers a woman who's a size 12 or 14 'plus size,' " stylist and TV personality Robert Verdi said. "I call that normal."

It was by accident, Matthew Kaness says, that he realized the women's apparel industry was in need of a shake-up.

Shortly after he took the helm at ModCloth in 2015, Kaness opened the company's first pop-up stores. Every day, visitors asked the same question: "Where's the plus section?"

"And the answer was, we didn't have one," Kaness said.

Instead, all sizes, from XS to 4X were grouped together, by style.

"People's faces lit up when they heard that," he said. "For a lot of them, that was the first time girlfriends, mothers and daughters, co-workers could shop together in the same store. That was the lightbulb for me."

A few months later, he removed the "plus" section from the company's website. In three years, Modcloth, which is now owned by Walmart, has nearly quadrupled its lineup of larger sizes.

That's been possible, Kaness says, because the company primarily operates online. It keeps all its inventory in one warehouse in Pittsburgh, which means it's not much of a problem to stock any given dress or T-shirt in nine sizes.

"We can constantly add sizes to every item without worrying about, 'How many extra-smalls and 2Xs should I send to the Tallahassee store for the third week of August?' " he said. "That's where e-commerce has an advantage in terms of serving a wider range of customers."

But, he added, there are extra costs and labor involved with offering more sizes. A size 6 dress pattern, for example, can be used to create dresses in sizes 0 through 12. But larger sizes require a second prototype, which means the company has to design two pieces, cut two sets of patterns, and fit them on two models.

"Even though online it looks like we're offering the same dress in size 2 or size 22, everything about those pieces is completely different," Kaness added.

Another trend-driven retailer, Eloquii, introduces two new collections to its website each month. Since 2014 (after it parted ways with the Limited), the company has cut production time in half, and is also filling its racks with trendy, sometimes edgy, apparel. Its best sellers include a high-heel, over-the-knee boot for $139.90 and a slim-fitted, ankle-length pant that sells for $89.90.

"The old-school thinking that our customer can't wear stripes or shouldn't wear large florals or, God forbid, a crop top to show her midriff - we don't believe any of that," chief executive Mariah Chase said. "We sell tons of off-the-shoulder tops. We sell tons of really-short shorts and crop tops."

But even as mainstream retailers - Walmart, J.C. Penney and Old Navy among them - expand up to size 30, or 5x, sizes larger than that continue to be neglected. Lane Bryant, Gwynnie Bee and Avenue all top out at size 32.

"It's really, really difficult to find fashionable clothing above 3X or 4X," said Debbie Christel, an assistant professor in Washington State University's apparel and textiles department who describes herself as a "fat studies scholar." "The thought within the industry is, if you want to wear fashionable clothing, you'll just have to lose weight. Not providing options for people because they're a different size is not only discrimination, it's absolutely absurd."

When Heidi Zak set out to create her own bra company, she was shocked by one discovery: Most of the country's bras had been based on the measurements of one woman.

Zak hired that woman - Dorothy Galligan, a 1970s cabaret singer who wore a size 34B - before realizing the approach was all wrong.

"I remember thinking, she's just one person," Zak said. "What about everybody else?"

Instead of taking one size - 34B - and sizing it up or down, Zak started from scratch, measuring hundreds of women of all sizes and recording their dimensions. She created a Fit Finder quiz, which is still the first thing customers encounter on the company's website, to gauge such things as body shape, height and breast shape. More than 4 million women have submitted their measurements, providing countless data points for Zak and her team to mine.

The results have been tangible, Zak says: ThirdLove, which started with cup sizes A through E, now goes through size K. The company also offers half-size cups.

"This entire business is data-driven," Zak said. "And ultimately it's about one thing: Being inclusive and serving all women."Chicago is among the top cities getting its party on. It ranks third in the nation when it comes to planning events throughout the year, according to Evite's third annual list. Check out what cities "set the bar when it comes to having fun and making time for family and friends," says Evite CEO Victor Cho.