Now let's hear it for the voluptuous and the mature.

Paris designer Karl Lagerfeld has started a search for models who are "more rounded" rather than super skinny. A recent cover of Gentleman's Quarterly focuses on a woman who could fairly be described as "zaftig." Beauty books pitched to a large sizes and mature women have started to appear and sell well, including Stella Jolles Reichman's "Great Big Beautiful Doll" and "Beauty for the Mature Woman" by Dorothy Seiffert.

And shortly, Helena Rubinstein will introduce the first group of treatment products geared solely for the woman over 50. Garfinckel's parent company has just purchased a group of 70-plus shops for the stylishly stout.

Yesterday the Hecht Company initiated a series of fashion shows and seminars for the mature or full-figure customer or both. Called Today's Woman, the first show played to an overflow crowd with many women standing on nearby stairs to catch a glimpse of the fashions.

While "mature" and "heavy" are by no means always related, Hecht's pitched its presentation yesterday to both groups.

This recent spurt of attention to these groups stems in part from stores, in a period of heightened competition for the consumer dollar, mining areas of business once considered marginal. And also because stores, manufacturers, advertisers and the like have become increasingly aware of the population shift to higher age groups.

The facts are clear. The elderly are increasing 3.5 times as fast as other age groups. By the year 2,000, if present trends continue, one in five persons in the United States will be 65 years or older, twice as many as there are in that age group today.

People are living longer, but what is influencing the figures more is that fewer babies are being born. The baby bust followed the baby boom. The number of births for each 1,000 American men in prime child-bearing ages in 1976 was 65, about half of what it was in the peak baby birth year of 1957.

So it is plain good business sense that has provoked stores, publishers and manufacturers to look to the mature customer to take the place of the declining percentage of children.

"Everywhere I go someone is doing something for the mature," says author Seiffert, who got yesterday's fashion show crowd doing exercises after the fashion show. "People are tired of the youth movement or the displays on models that are very, very slim. They are beginning to realize the power and the money of older women."

At least one clothing manufacturer has changed the tone of fashion advertising for the large woman. Reads the Legacy advertisement in a Sunday newspaper: "It's about time somebody spoke up for the real world.The world where men respond to women for a lot more important reasons than the narrowness of their hips. The world where a woman gets what (and whom) she wants because of what she accomplishes and how she thinks of herself . . ."

Such ads, plus fashion show like that at Hecht's, are developing demand for the more mature and full-figured models. The Barbizon agency, for example, is training and sending out on jobs more size 14 and over models than it ever has before.

Seiffert is critical of the way many older women dress. Last week on a cruise to Bermuda, many of her deckmates, she says, were in pale pink and baby blue polyester pantsuits, "pastels rarely look well with a mature woman's usually sallow complexion," says Seiffert. And a softer fabric is more flattering than polyester knit.

She's also down on faddish clothes, a tiny all-over prints. "They make you look lumpy," says the trim 66-year-old former model.

Rosamonde Niemeyer, from Hecht's fashion office, showed clothes at the show she felt, were most appropriate for the bigger woman. They included blouson tops and matching skirts in challis, tie-front sweater vests, lace-trimmed blouses, clasticized stright-leg pants and skirts, several of the currently popular peasant looks.

In general she offered these tips: Shop with the proper foundation; wear comfortable shoes; don't buy anything top tight; don't be afraid of bright colors; don't buy anything too short." "There is nothing uglier than the back of the knee," Niemeyer said. "And check your profile in the miror," she added.

Hecht's is doing its best to keep yesterday's event, ice cream sundaes, these customers pleasingly plump. At with hot fudge, nuts and strawberries were served on the house.