I don't really know if this would be the best solution, but we could maybe define a sliding LoC limit per PR? Also a limit on the number of open PRs per contributor. This way those small, reviewable contributions would get a fair shot at being landed instead of being blanket banned, and the maintainer overhead would be more reasonable.

Even that small one-line contribution is not the contributor’s own work. Since the contributor does not hold the copyright to it, they do not have the right to contribute it to the project.