The miniaturised people theme has always held a certain fascination in genre material from the various versions of Gullivers Travels (1726) to films such as Dr Cyclops (1940), The Incredible Shrinking Man (1957), Fantastic Voyage (1966) and Honey, I Shrunk the Kids (1989) to tvs Land of the Giants (1968-70). Miniaturisation is not exactly a common theme the only major miniature people film we have had in the last three decades has been Marvels Ant Man (2015).
The miniature people theme always comes with some a number of plausibility issues from a scientific viewpoint. Assuming it would be possible to reduce a person to about one-fourteenth of their size, all of their body mass has to go somewhere. This leaves either of two options removal or compaction but either of these present significant problems. Removal of body mass also means removal of brain tissue meaning that reducing someone the size of dormouse or a frog would only leave them with the equivalent brainpower. The other way to go would be compaction but increasing the density of someones body mass so that they would inhabit a smaller space would mean substantially increasing their weight within a smaller area such that they would probably leave dents in the ground with every footprint. The other issue coming in to play is that while the person would be much smaller, the laws of physics would still act normally. We do see this sort of in the scenes in Norway where the boats wake on the fjord looks unnatural, which you take as an attempt to approximate the way water would move around a model boat. However, there would be other issues too such as the question of whether someone at miniature size would still be able to process food and air when the molecules would suddenly be a whole lot larger than a normal body is used to dealing with.
The film also posits the idea of miniaturisation as a solution to consumption issues around the world. For Matt Damon and Kirsten Wiig, miniaturising presents a solution to being able to afford to buy a decent home. The thinking that being smaller means using less resources therefore money will go a lot further seems naive economics at best. You suspect that if such a reality came to pass then all that would happen is corporations rushing to take advantage of the small boom and inflating prices someone has to design all the miniature houses, furniture, clothing and process the food, for example. The lack of people able to perform precision miniature craftsmanship would be scarce, making the cost of such goods very high at the outset.
One should always be wary of a non-genre director deciding to dabble in genre material usually accompanied by much insistence that what they are making is not a genre film (which only reads as them considering genre association as beneath them). Nevertheless, Alexander Payne presents the premise to us straight up and has everyone accepting it with matter-of-fact calmness greeting old friends that have gone small with fascination, goodbye parties held for those undergoing the treatment, sales pitches and real-estate meetings about purchasing a house that is the essence of good science-fiction. On the other hand, after introducing the premise, the film slows down as soon as it gets to Leisureland. All of the other abovelisted miniaturisation films have the little people engage in a series of adventures in the big peoples world and hold the fascination of seeing them trying to navigate or use big objects. Here however, after Matt Damon is shrunken, Payne shows almost nothing more of the big world one scene of Damon signing his divorce papers, another with him carrying a giant-sized rose, his venturing into the shantytown built around the inside of workers huts and some full-sized butterflies at the end, but that is it. It seems almost an abandonment of all the interesting possibilities of the films premise.
Downsizing also confounds you when you keep trying to think of it in terms of a genre film. It raises plot elements that you think any other film would have taken and run with but instead goes completely sideways with them. One of these is the introduction of the End of the World in the latter sections. Almost any other film would have created a big drama about racing to find a solution or regarded the settling into the shelter as the climactic hope for the future. By contrast here, Alexander Payne raises the End of the World off-screen but then only uses the shelter as a way for Matt Damon to have to make a choice between the future and the love right in front of him.
Equally, the film takes a fascinating mid-film twist where Matt Damon becomes aware of a whole social and ethnic miniaturised under-class living in a shantytown beyond the walls of Leisureland. The film has been described as satire by a number of reviewers but all I came away asking is exactly what these other commentators thought it was that the film was satirising (usually in satire there is some clear target that is being lampooned). Another film might well have become indignant about the social disparity, about how contemporary social divides have been copied over to the miniature world, but Alexander Payne doesnt seem interested in exploring any of that. Contrarily, what he does seem to be interested in is creating another relationship drama initially the comic one of Matt Damon being forced to work as Hong Chaus slave after breaking her artificial leg and then a not-entirely-convincing one where he suddenly develops feelings for her. Here Matt Damon plays another variant on the typical Payne protagonist who discovers a renewed sense of purpose after undergoing a mid-life upheaval. Certainly, what you commend the film for here is Hong Chaus performance where she manages to give all of her scenes a live wire determination and often plaintive emotion despite her performance being delivered through broken English.