Thanks for taking the time to be thorough in your request, it really helps! 
The backups doesn’t just include git data, but also stuff like CI/CD job artifacts and container registry images, so your comparison is a bit off.
How long do you save backups? I.e. What is gitlab_rails['backup_keep_time'] set to in gitlab.rb?
If you look at how the backups work, not much can be done about a lot of the usage, but now (since the release of 12.9, so you’ll need to upgrade to use it) there is one thing you can do: you can use SKIP=tar, to skip the creation of the final tarball, that reduces the amount of space used during the backup process. But it also means you can only store one backup on the gitlab server.
You might be able to learn something from reading the documentation:
https://docs.gitlab.com/omnibus/settings/backups.html
https://docs.gitlab.com/ee/raketasks/backup_restore.html