Photo: Toronto International Film Festival

In Alexander Payne’s toothless science-fiction comedy Downsizing (Grade: C), living large means living small. Ten years after the invention of a miracle technology, designed to solve the planet’s intensifying overpopulation problem, people begin shrinking themselves down to around five inches tall—though not, mind you, out of pure altruism. This is the new ticket to a life of luxury. Because everything is so much smaller, money goes much further: Any middle-class wage slave can afford a mansion the size of a dollhouse—you just have to be little enough to fit inside of it. It’s like that timeshare you got suckered into buying or that vacation house the bank never should have loaned you the money to purchase. Except, of course, instead of mortgaging your future for the illusion of wealth, you’re willfully reducing yourself to the size of a rodent to live how the other half lives.

Payne generally specializes in lower concepts, in a drabber species of Midwestern tragicomedy, and it’s tempting to simply salute his willingness to stray outsize of comfort zones; the director of Sideways and Nebraska has made a bona fide special-effects movie. But Downsizing is less a fully-formed satire than a clever idea stranded in first draft and stretched uncomfortably to feature length. The film’s best joke is its premise, a metaphor for the insane lengths people will go for material possession or even just the feeling of upward mobility. But Payne almost seems embarrassed to take the idea any further, to really exploit the visual or conceptual possibilities of a world of tiny people. Instead, he just trots out another Middle-American sad-sack schlub, a literal small man for once, played this time by an admirably ordinary Matt Damon.

In truth, Downsizing never really decides the movie it want to be; it bails even on its sketch-comedy conceit. Damon’s character, an occupational physical therapist named Paul Safranek, takes the leap and relocates to a kind of miniature gated community called Leisure Land Estates, where he ends up living below a rich hedonist played, with the usual lack of restraint, by Christoph Waltz. Here, Payne’s sentimental streak takes the wheel. Paul, alone and regretful about his decision, ends up bonding with a one-legged Vietnamese dissident (Hong Chau); Payne plays her broken English for broad, casually racist humor, while also supplying her with not one but two bathetic tearjerker monologues. (This is the same filmmaker, remember, who turned “Dear Ndugu” into a cheap punchline.)

Damon, who’s also appearing at TIFF this year in George Clooney’s Coen-penned Suburbicon, supplies a certain bruised dignity. (One thinks, vaguely and occasionally, of Behind The Candelabra, where the star played another dejected man wrestling with his regret about an irreversible physical transformation.) But the film never lets Paul’s existential life crisis breathe; instead, it zigzags off in a gooier direction, into an earnest eco drama about one’s place in the fight for the planet. Downsizing doesn’t just squander its magical-realist premise. It kind of forgets about it. Maybe that’s Payne’s seriocomic point: that even an impossible development like voluntary shrinking would become just another facet of modern life, with its haves and have-nots. But it’s still hard not to daydream about what a filmmaker with, well, a bigger imagination might do with this outlandish idea.