The Initial Block Download (IBD) on macOS Catalina uses a lot more disk space than expected.
The macos manpage for fcntl (for F_PEOFPOSMODE) states:
Allocate from the physical end of file. In this case, fst_length indicates the number of newly allocated bytes desired.
This would result in the rev files being essentially pre-allocating 2x their necessary size (this is the case for block files as well, but these are flushed down to their right sizes every time) as they would pre-allocate pos + length free bytes, rather than allocating length bytes after pos, as expected.
Note: more explanations on the issue from Apple in this bug report
Backport of Bitcoin Core PR17887