A scientist has created a way to solve overpopulation: shrinking people down to a fraction of their size and having them live in small communities. The movie shows a man (Matt Damon) at a company lecture and consulting with a realtor for a home in a small community. The interaction between the two demonstrates the personal and social tradeoffs of a positive externality. The family shrinking themselves down is doing it for personal reasons, namely to save on living expenses and to no longer work, but there are social benefits at play as well. Having everyone shrink to a fraction of their size can help alleviate overpopulation and reduce human waste.
Thanks to Amanda Yaya for the suggestion