AOSP is probably the closest comparison, Fedora/RHEL and Ubuntu don't keep application source code checked into git. Fedora specifically uses dist-git, there's a git repo per package with the spec file, patches and other files needed for the build and then source tarballs are pushed to a web server where they can be downloaded later with the dist-git tooling.

So yeah, all of the code and data actually stored in source control for various Linux distributions is pretty small.