Cloud accounts · Cloud apps on your Mac
Should I use Stream or Mirror in Google Drive for desktop to save space?
Short answer: Stream, if space is tight. Streaming keeps files in the cloud and downloads them when opened; Mirror keeps a full local copy of your whole Drive.
Choose Stream if your Mac is short of space. Streaming shows your whole Drive in Finder but keeps files in the cloud, downloading each one when you open it. Mirror keeps a full copy of your Drive on your Mac as well as in the cloud, so it uses as much local space as your Drive holds.
Mirroring suits people who need every file offline all the time. Streaming suits most laptops: you can still keep particular files or folders available offline from Finder's right-click menu, and only those take space.
To check how much Google Drive's local copies are taking, open Preferences → Rules → Free Up Space includes in Storage Studio — each cloud provider on your Mac is listed with its local size. See freeing Google Drive's space.