I don't know it has as much to do with scale of the system vs the general architecture. E.g. the system I primarily work with these days has millions of lines but most PRs are for a small changes which are well contained in scope by the overall architecture.
It's a cultural thing, but you can do incremental PRs towards a large goal. Giant PRs that are expected to be reviewed never really seemed worth it imho.
I don't think a PR needs to be tens of thousands of lines for a review to take more than 20 minutes. I've worked on projects where you can't reasonably get a branch set up and running somewhere to test it within 20 minutes if you need to set up dependencies, peripherals, external systems, etc.
reasonably you should already know what the code and project is supposed to do before they even start writing code, so you can get directional feedback in.
then you are maybe reviewing 1 out of 7 PRs that implement the agreed upon change
