Some of America's biggest ready-to-wear customers don't buy a lot of clothes, says Lee Birmingham, store manager of Woman's World, a specialty shop pitched to the large woman at Lakeforest mall.

The large-size consumer, she says, is "convinced that next week she's going to be a bit slimmer, so she just buys to suit her immediate needs."

Nonetheless, big women are big business, as Aileen Knapp from Sears -- regional director of Washington Fashion Group -- pointed out at a luncheon at the Woman's National Democratic Club yesterday.

More than 25 million American women are size 16 or over -- 8.5 million of them over size 22 -- according to figures compiled by Fashion Group, an international educational organization of women in the fashion business. Thirty percent of the adult female population fits in this large category -- one out of five of them under 35.

In spite of the sizeable business potential, more than three out of five of these women find it difficult to get proper fit in clothes. And two out of three think manufacturers simply don't care about heavy women. Most would be pleased to see clothes displayed on hefty models.

That is exactly what happened yesterday as guests at the lunchcum-fashion-show saw clothes from Lane Bryant, Montgomery Ward and Woodward/Lothrop shown on Rubenesque models, sizes 14 to 18 1/2. Slides also pointed out the range of clothes available in large sizes including the current crop of general fashion favorites: slim skirts, shaped jackets, belted styles and bright colors. Styles for large women by designers Giorgio Sant Angelo, Kasper, Adele Simpson, Mady Gerrard and Vera Maxwell were shown on slides.

Dr. Bennetta Washington, guest of honor at the luncheon -- who was wearing a chemise-style dress by Hanae Mori -- said she has attended a conference on the problems of large-size women at Lane Bryant in New York.

"Many young women who are large have the problem of not feeling adequate and self-sufficient because they don't look like models," she said.

Washington, who "started out being quite slender" said that being a large woman has not handicapped her. "You can't let it be a barrier in terms of your concept of yourself," she said. "You have to appreciate your own uniqueness."