Baryshnikov in a charming Twyla Tharp ballet

Yes, that old saying about good things coming in small packages is true. Of all the premieres offered by American Ballet Theatre at the Metropolitan Opera during an engagement that runs through July 16, the most unassuming turns out to be the most substantial.

Twyla Tharp's ''Once Upon a Time'' slipped into the repertory practically unannounced, as if an afterthought. Although it stars Mikhail Baryshnikov, who is a star if ever there was one, ''Once Upon a Time'' doesn't treat him as a star. Tharp maintains a miniature air for her ballet, but there are so many lovely things about it that it can well afford its modest stance.

''Once Upon a Time'' is a perceptive modernization of romantic ballet. Tharp usually makes dances to heavyweight composers drawn from the best of jazz and classical music. Tellingly, here she uses selections from Alexander Glazunov, who is not great in the standard sense, but nevertheless supported formidable choreography in 19th-century Russia.

Tharp uses Glazunov's lyric waltzes at their face value, which is valuable enough, for it inspires amplitude and graciousness. Yet being musically sophisticated, Tharp has also thickened Glazunov with choreographic syncopations. But one example of the ballet's care and charm occurs when the three-woman ensemble trickles off the stage in between notes, as a delicate doily is laid over a tablecloth.

These three women function as three graces to the cavalier (Baryshnikov) and his lady love (Deirdre Carberry). As in a conventional romantic ballet, their presence heightens the mysteriousness of the romance between the couple in love. Tharp further heightens this fugitive air by bringing on the ensemble at totally unexpected times and then whisking them away capriciously.

The ballerina is elusive, as all romantic ballerinas are, and at one point the cavalier goes in search of her. But Tharp's version of the old story has wonderful twists. Rather than stroll about the stage in search of the sylph, Baryshnikov races around in a vernacular sprint. Yet he is more elusive than the women he chases, for at the heart of the ballet is a series of solos in which everything Baryshnikov does is off-balance and askew. Tharp often works in out-of-kilter shapes and unexpected thrusts. In ''Once Upon a Time'' these Tharpian devices have a new and remarkably insightful point.

One of the models from which Tharp's ballet springs is Bournonville's ''La Sylphide.'' ''Sylphide'' also happens to be the biggest new production of American Ballet Theatre's season. It's staged by Erik Bruhn.

Bruhn's production doesn't have new things to say about the classic; it just says them in a big way. As played by Bruhn himself, never has the witch Madge been more malevolent. Nor has the decor, designed by Desmond Heeley, been more elaborate. The farmhouse from which the sylph lures James away from his fiancee is positively palatial, and the woodland glen where they meet their destinies is spectacularly idealized. The sylph couldn't expire in a prettier way. She ascends into sylph-heaven through a rainbow of mist arching high across the stage. Three cheers for stage machinery!