In Downsizing, director Alexander Payne comes up with a new breed of Lilliputian. Standing just short of 13cm, they're "downsizers": people who have elected to reduce themselves physically in the hope of enlarging their assets and raising their expectations.

The bonus is a licence to feel they're doing their bit for the future of the planet by cutting down on their consumption of its resources.

The whole idea makes a great launching pad for a missile attack on an assortment of society's ills, and Payne (Sideways, Election, About Schmidt) seems just the person to take it on. The strength of his movies has always lain in his ability to compress big themes into intimate stories, lit with sharp and sometimes poignant insights about the vagaries of human nature.

The audience at my screening certainly had high hopes; eager to be amused by the mere thought of people choosing to be shrunk to the dimensions of Barbie and Ken dolls just to improve their real estate prospects, they started laughing as soon as the titles stopped rolling.

Real estate is what it comes down to when Paul and Audrey Safranek??? (Matt Damon and Kristen Wiig) are first introduced to the idea of downsizing by an old friend of Paul's, who startles everyone at their college reunion by appearing in his new, miniaturised form. Having made the change, he and his wife are now residents of Leisureland, a luxuriously appointed toy town where the Safraneks' paltry savings could buy them a whole estate - complete with a mansion and broad expanses of green lawn.

After giving insufficient thought to it they take the plunge. Paul is full of optimism - until he wakes up in his new body to find that Audrey has lost her nerve at the last minute and gone home to Omaha.

The phone call in which Wiig delivers this piece of news is such a neat distillation of pure panic and deadpan comedy that you're sorry to see the last of Audrey. And so is Paul.

From this point on, Paul's life is transformed into a long, winding and increasingly unpredictable road as he becomes thoroughly acquainted with Leisureland's downside. It, too, has its have-nots - and he's now one of them.

He has a boring job and he's left the mansion - which had become a prison without Audrey - for an apartment. Things are very dull until he gets to know his neighbour, Dusan, a conman and party boy who has put a lot of effort and cunning into making Leisureland work for him.

Played by Christoph Waltz, who is as funny as ever, Dusan has a glittering eye for the main chance. He's an ethics-free zone and finds great entertainment in Paul's possession of a moral compass.

Paul's horizons begins to expand in unexpected ways. He meets Dusan's cleaner, Ngoc Lan (Hong Chau), who has been downsized against her will as punishment for her activities as a Vietnamese dissident. Paul and Ngoc Lan set about helping Leisureland's immigrants, who have been exiled to a slum on the other side of a wall circling the town.

Berlin, Israel and Trump's Mexican wall all come to mind. So do the dangers of Paul's isolationism: neglect your responsibilities towards the rest of the world, the film is saying, and you become smaller in every way.

Finally, there's the paradox at the hub of the whole downsizing concept - which is a product of socially conscious Norway. Conceived by a group of idealistic scientists who have proved their commitment by downsizing themselves, this supposed reform was supposed to downsize the world's population over the course of 200 years. Instead, the concept is rapidly turning into an instrument of oppression.

It's a plot bristling with so many metaphors that Payne can't make up his mind about the best way to handle them, and it's not long before the whole thing begins to lose tension and momentum. The Disney-esque aspects of the story soon faded, as did my audience's initial urge to laugh.

Yet there are some droll moments - most of them supplied by Hong Chau, whose Vietnamese dissident is so stridently matter-of-fact in her acceptance of human tragedy that the mild-mannered Paul is mesmerised by her. And she's no fatalist; along with her matter-of-factness comes a fierce urge to get on with the job of helping those who need it.

It's a tantalising film - a picaresque tale packed with big ideas and original ways of looking at them. Yet Payne fails to find a rhythm for it, or a tone consistent enough to tie its disparate elements together.

I wanted very much to like Downsizing but ultimately, I found it a long, slow haul.