Piglet's Big Movie, like 1977's seminal ''The Many Adventures of Winnie the Pooh,'' is a collection of shorts, here presented as flashbacks. All three derive from A.A. Milne's original tales, but retain only a smidgen of his droll, easy-chair wit. (Carly Simon's tunes are no substitute for a good narrator -- Sebastian Cabot, you are sorely missed.) The cause for all this reminiscing is a search for the wee porker (voiced by John Fiedler, hamming it up in this role for 35 years now), who has wandered off dejectedly to ponder his feelings of inadequacy. The drama culminates in a literal cliff-hanger, which (a) feels too typically cinematic for the tempest-in-a-honeypot scale of the Pooh-niverse, and (b) raises some puzzling topographical questions about the Hundred Acre Wood.