When I have problems like this, the best approach is to:
- open Finder
- Go to your root directory (Macintosh HD, in most of the cases)
- Chose "list" mode
- Press Command + Shift + . (or the period)
- Click View > Show View options and click Calculate all sizes
- Click on the "Size"column to order from large to small
- wait...wait...wait.
This will, slow, start showing how much space is in use by each directory. You can click the ">" on the left side to show sub-directories. Ignore System and Library directories.
With this information in hands, you can identify where the space of your Volumes is gone.
Remember that Documents is a link to iCloud. The space used by it can be adjusted to offload files if necessary.
This is how one of my computers looks like.