In DO180 (v4.6), Chapter 4, Section 3 there is this Note -- that probably can be removed or, at least, updated to reflect what the man page says:
"Compress tarball image layers when pushing to a directory using the 'dir' transport. (default is same compression type, compressed or uncompressed, as source) Note: This flag can only be set when using the dir transport i.e --format=oci-dir or --format-docker-dir"
(I just noticed the typo in the man page: --format-docker-dir should be --format=docker-dir)
The note suggests that the compression can be used on its own - it cannot. Well, it can, but it doesn't make any difference.
As I'm the curious one, I tested it out. Whether --compress is used or not, the file size is the same because the --format option, with 'dir' transport, was not used:
When --format is used, the result is a much smaller footprint:
