(replying to the entire list instead of Bruce only this time...)
This doesn't make sense to me. Since we hard-linked, why would 12 be so much smaller? If it was symlinks, I could imaging that, but it doesn't use symlinks, just hard links, so it should be similar. Please look at the size of main/base on both, since that is where your data is. Maybe 11 just has a lot of old WAL that isn't copied.
It depends on how the directory sizes were measured. "du" won't count the same file more than once, so if space was measured using one of the following commands:
du -hs /var/lib/postgresql/
du -hs /var/lib/postgresql/11/main /var/lib/postgresql/12/main
Then you would see a large value for the 11 directory and then the 12 directory would only show the files that weren't already counted when measuring the 11 directory. Running du on each version's directory separately might give results that are more in line with expectations.