Ever since DriveSpace on MS-DOS (or really, Stacker), we've had transparent file compression, with varying degrees of automation; in fact, while the DriveSpace-compression on MS-DOS was a fully automated affair, the built-in transparent compression in newer filesystems such as ZFS, Btrfs, APFS (and even HFS+), is engaged manually on a per-file or folder basis.

But no one's using it!

On my system, compressing /Applications saved 18GB (38.7%).

MacOS doesn't actually come with a utility to do this even though the core functionality is included, so you'll need to install an open source tool in order to use it.

$ brew install afsctool

To compress a file or folder, use the -c flag like so:

$ afsctool -c /Applications

(You might need to use root for some application and/or system files).