Cloud accounts · Cloud files and your Mac's space
How can I see how much space Google Drive, Dropbox or OneDrive take up on my Mac?
Short answer: In Storage Studio, open Preferences → Rules → Free Up Space includes: each cloud folder on your Mac is listed with the space its downloaded files really take. Finder's Get Info counts online-only files too, so it overstates it.
Open Preferences → Rules → Free Up Space includes in Storage Studio. Under Optimise cloud storage locally, every cloud folder found on your Mac is listed with how much space its downloaded files take, plus a total of how much of your internal disk is caching cloud files. Use the refresh button to measure again.
Don't rely on Finder's Get Info for this: it reports the full size of every file in the folder, including online-only ones that take almost no space, so the number can be far larger than what's really on your disk.
Storage Studio's Storage Profile also lists big downloaded cloud files among your biggest space users, each with a Free up button. See what Free up does.