Here's another interesting observation. For previous versions, I was able to reduce the size of the updates mirror significantly by running repomange -o and deleting the resulting files on next sync. With the AppStream repo, this no longer works, the anaconda installer will complain that at least one package has an incorrect checksum, and the only way to remove this error is to restore the complete package set, multiple versions and all. I've not been able to narrow it down more.