Mark Westcott won the first University of Maryland piano competition 11 years ago, and he returned last night to display what has become of him. Sometimes the apparent potential of the big prize winners materializes and sometimes it doesn't. Even a Cliburn prize winner may not make it in a major career.

Artistically, Westcott has made it. In fact, given the authority of his playing last night in Beethoven's epic, almost hour-long "Diabelli" Variations, one wonders why Westcott's name is not more famous.

These 33 variations on a frivolous little waltz tune of the day may well have been the last important keyboard work of the man who is arguably the greatest of all keyboard composers. Its scale--emotionally, intellectually and technically--almost beggars description. You name the mood, and it's there. The metamorphoses of that little tune are incredibly free. There is the lusty martial opening variation, for instance, or--astonishingly--the 22nd variation, which still follows the melodic formula but whose tune turns out to be "Notte e giorn o faticar" from Mozart's "Don Giovanni," or the lofty final variation, regarded by Schumann to have been the composer's farewell to music. Only one work remotely springs to mind as comparable, and that is Bach's "Goldberg" Variations.

Westcott's command of all this creative glory was remarkable for a pianist at any stage of his career. The pulse was utterly steady, but the music breathed easily. The variety of mood was laid out graphically. Also outstanding was his grasp of contrasting sonorities.

He shared the program with the second-prize winner from 1971, Diane Walsh. One does not wish to belittle much of what she did, but her finest moment came in the brief encore, an A-major Scarlatti sonata, which was crisp and succinct.

Otherwise, in Mozart, Mendelssohn and Debussy, she showed more digital agility than musical. Her performance of Mozart's grave A-minor Sonata had the necessary expressive agitation, but not the singing line. And that was true of much of her playing.