Server space full but nextcloud takes just 3/5


I choose an provider ( to host my nextcloud. In general it works fine.
My problem is that my 50 GB server capacity is full while my nextcloud only uses approximately 30 GB (all users together). It is my only application on this server.
I cleaned my deleted files, but that didnt help.

Do you now if nextcloud keeps some files elsewhere that need such big space?
Or: Do you know how I could find out what it is in general that keeps my server full?

Thank you for your help!