I have a 5 GB site, years old, and I know lots of the images in uploads are not used by any current pages or posts.
A 5 GB site is tough to backup.
There are the DNUI and Image Cleanup plugins, but 25-50% of the reviews are 1 star because the plugins deleted actual used images. Apparently it’s a tough problem – unattached doesn’t mean unused, and themes and plugins point to images on disk in a way that these plugins don’t find when trawling the database.
Seems to be a common problem, too.
Opportunity for wpmudev?