If WAL compression is enabled, does this mean that traffic to a file-based WAL-shipping standby is compressed? (In other words, WAL files are sent post-compression?)
From the docs:
When enabled, the PostgreSQL server compresses full page images written to WAL when full_page_writes is on or during a base backup.
That is, some of the contents are compressed, it is not the container that gets compressed. That is always 64MB in a default compilation.