Note: please do not manually delete the files from Steam's workshop content folder. Steam will expect them to still be there and will re-install them when verifying game files.
When you subscribe to a workshop file or join a server which downloads workshop files Steam will keep them installed for a certain amount of time regardless of whether you unsubscribe. This is (presumably) so that that if you re-subscribe or a server uses them again soon they don't need to be re-downloaded. Only subscribed files are loaded during startup, so there is no performance penalty to having the non-subscribed files installed.
At the time of writing (2023-06-02) Steam doesn't delete the per-workshop-file folder when uninstalling content. This may make it appear as if more content is installed than there actually is.