In a new definition of economy of scale, Alexander Payne’s “Downsizing” envisions a radical solution to overpopulation: shrinking people to 5 inches tall. It’s certainly one way to prevent climate change and resource depletion: Volunteers will literally leave a smaller footprint. The real attraction, though, is that even a modest nest egg can buy a life of luxury in the mini-community of your choice.

Who wouldn’t at least consider it? That’s the brilliance of “Downsizing,” which constructs its fanciful premise with such inventive detail (teeth won’t shrink, so they’re pulled and replaced) that it becomes utterly plausible. We see everything through the eyes of Paul Safranek (Matt Damon), an Omaha physical therapist living a semi-comfortable existence with his wife, Audrey (Kristen Wiig). When he discovers that an old buddy (Jason Sudeikis) has “gone small,” Paul sees a new way to achieve his American dream.

Right when Paul takes the plunge and begins his tiny new life, however, something unexpected happens: “Downsizing” runs out of ideas.

What? Just as our hero is entering a new and potentially fascinating world? Sadly, yes. Payne and his longtime co-writer Jim Taylor (“Sideways”), immediately begin stabbing around for a story. Paul’s neighbor, Dusan Mirkovic (Christoph Waltz), proves a colorful character, a graying party animal and prosperous smuggler, but he provides neither conflict nor catalyst. He’s amusing, but a narrative dead end.

A more promising, if slightly dubious, character emerges in Ngoc Lan, a Vietnamese immigrant whose pidgin English and brusque demeanor teeter on ethnic stereotype. (She’s played with spirit and sharp timing, though, by newcomer Hong Chau.) Ngoc, a housecleaner, serves as Paul’s entree into that most exotic of subcultures: Poor People. Paul is stunned to find they exist, but he doesn’t become an activist or a crusader. Instead, the movie sends him and Ngoc on a long journey to yet another small community, one that has become a kind of doomsday cult.

Even as “Downsizing” drifts into the narrative weeds, it poses some very big questions — about the earth, mankind, the future and what makes a life worth living. Chalk this movie up as a bold experiment that achieves mixed results.

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‘DOWNSIZING’

2.5 stars

Cast: Matt Damon, Christoph Waltz, Hong Chau.

Rated R (language and mini-nudity).

Running time: 2:15