Although big balloon sleeves and bustles are not what you would call modern, they do have their adherents in today's world of design. Some fashionmakers have been backing up into the 19th century lately with more verve than they show in moving along with the 20th.
Yves Saint Laurent may be the responsible party. He set the pace with his rustling taffeta Proust dresses a few seasons ago, and before you could say ''wasp waist,'' many of his colleagues were also flirting with the sort of hourglass silhouettes that call for coachmen in livery and a foursome of chestnut bays.
We must not expect that this time warp will disappear soon into fashion's never-never land. The splashy new exhibition in the Costume Wing of New York's Metropolitan Museum of Art (''La Belle Epoque,'' on view through Sept. 4, 1983) is chockablock with elaborately comparisoned turn-of-the-century figures. They appear to wear corsets, and they often carry trains that drag on the ground behind them.
The show is the eleventh annual extravaganza to have been produced by Diana Vreeland, the remarkable fashion personality who, as special consultant to the Costume Wing, has made that department of the museum take off and fly.
The theatrical flair Mrs. Vreeland brings to her exhibitions has in past years been a magnet for tens of thousands of visitors.
People who remember the movie ''Gigi'' and Lehar's ''Merry Widow'' will know the belle epoque as the era when considerable fun and frolic went on nightly at Maxim's.
An evocation of such legendary doings forms a part of the exhibition. Portions of the art-nouveau-mirrored gilt-and-mahogany interior of the famous Paris restaurant have been re-created as a backdrop for female mannequins in overdecorated satins and velvets, accompanied by dashing escorts in top hats and opera capes.
Reproducing Maxim's was made possible, as they say on public television, by a hefty grant from the Pierre Cardin Management Corporation, which also funded another more-ambitious Maxim's-at-the-Met. The museum's cavernous restaurant was transformed for the night of the gala benefit-opening party into a huge replica of the Paris original. Then the whole shebang was ''struck,'' like a film set, and carted away the next morning.
''Why talk of money?'' said Pierre Cardin, when asked whether $300,000 or $ 500,000 was the fairer of the reported estimates of his contribution to ''La Belle Epoque.'' Mr. Cardin, whose world network of licensed products has made him the richest couturier of them all, said that many of the fixings, including the candelabras with their pink silk shades, would be utilized in his soon-to-be-opened Maxim's in New York. It will be the eighth of the international chain of Maxim's owned by his corporation.
Using a commercial tie-in to bankroll an expensive show when it is staged within the framework of a museum might raise a few eyebrows. But Mrs. Vreeland's exhibitions are not the usual museum fare. The public delights in them because they are entertainments (although not labeled as such) and are nonacademic to the point where scholarship goes out the window.
Besides Maxim's, segments of the exhibition a social historian will recognize as French belle epoque include some vignettes featuring the kind of summer whites appropriate for the Bois de Boulogne. These are face a face with other visions in lace and eyelet embroidery that were worn by their former owners in Newport, R.I., during America's gilded age.
Two galleries are devoted to resplendent ball gowns and court dresses that belonged to England's Queen Alexandra, Queen Margherita of Italy, and other notables of the day. The jeweled purple costume Sarah Bernhardt wore as Gismonda was not so memorable as the yellow satin gown decorated with black scrollwork, or ''ironwork,'' by Charles Frederick Worth, the father of Paris couture. The dress was made for Britain's Lady Decies.
Two Worth dresses illustrate the extravagant but exquisite taste of the Countess of Greffuhle. A social lioness who was the leading clothes horse of her day, she served as a prototype for Marcel Proust's ''Duchesse de Guermantes.'' She was born Elisabeth de Chimay, but she is called ''Helene'' on the placards with which the museum identifies her sumptuous cloth-of-gold gown, with a trailing skirt hemmed in sable and her black velvet dress embroidered down to its sweeping train with a pattern of silver lilies. The enduring record that photographer Paul Nadar made of her in this dress portrays to perfection her exalted position as well as the sinuous lines of the times of art nouveau.
Among other exhibits, there is a waxwork figure of Queen Victoria wearing her black shawl (on loan from Castle Howard), looking as she might at Madame Tussaud's. In a further confusion of periods, a room filled with post-belle-epoque clothes by such stars of the pre-World War I couture as Paul Poiret and Paquin completes the show.
Enjoyable as long as it is not taken too literally, ''La Belle Epoque'' may, for all we know, revive the corset industry, put women in bustles, or inspire today's hosiery manufacturers. One of the smaller exhibits - Queen Alexandra's black stockings, which have bright silk flowers embroidered up the instep - is pretty irresistible.