People in the large-size clothing industry do not take offense to jokes about doing "big business." They are laughing all the way to the bank.

While the apparel industry in general has been struggling the past few years, designers, manufacturers and retailers catering to stout women have enjoyed steadily rising sales and profits.

Between 1991 and 1993, sales of so-called plus-size women's fashions--Size 14 and above--grew 16%. The increase, analysts say, reflects the reality that larger women are a significant market segment and are increasingly willing to pay more for clothing. The average American woman is 5 feet 4 inches tall and weighs 143 pounds, according to national health statistics. The apparel industry estimates that one in three American women, or about 35 million consumers, wears a Size 14 or larger. All of this translates into many more Size 18s being sold than Size 8s.

Encouraged by demand for stylish apparel in larger sizes, upscale retailers such as Saks Fifth Avenue and Nordstrom have created departments that cater to big women.

Merchandisers and sales representatives "are finding out that we are not just housewives looking to buy a couple of blouses and a few pairs of polyester pants," says Maryanne Bodolay, meeting and merchandising coordinator for the National Assn. to Advance Fat Acceptance, herself a Size 32. "We are doctors and lawyers and businesswomen."

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Nordstrom launched its Encore department for plus-size women in 1991 at its flagship store in downtown Seattle, and it has since included the section in 45 of its 78 stores nationwide.

"We've added that department because our customers asked for it," says Marie Joyce, spokeswoman for the Los Angeles Nordstrom stores. "The customer response has been incredible."

According to NPD Group Inc., a marketing and research group in New York, sales of women's plus-size clothing last year was about $14.4 billion, or 21.4% of $67.6 billion in women's apparel sales.

That's a lot of spending, analysts say, by women the fashion industry has historically ignored.

"For years, large-sized women had to go to Lane Bryant or Lerner Woman, or they had to rely on dressmakers and patterns," says Alan Millstein, publisher of Fashion Network Report, a newsletter for clothing retailers. "The industry took the consumer for granted."