/me goes to reduce the size of the volume back to a perfectly sufficient 10 GB...
Oh, bollocks. You can't decrease the size of Hetzner Cloud Volumes, only increase them. So I'd have to create a whole new one and then and then and then...can't be bothered with all that so I guess I'll just stomach the slightly increased cost of 15 GB over 10 GB.
That's "just" a difference of 2.66GB (which I guess could mostly be just Nextcloud files themselves)
So, assuming that is mostly just Nextcloud files themselves (I may do a clean install at some point just to see how big that is), I think this can be marked as solved.