Storage only shows "Other" + Fluctuating storage
I wonder if your Spotlight index is corrupt or Spotlight is turned off for the volume, and thus classifying all of the files as "Other". Try rebuilding the Spotlight index.

You can tell the Finder List View to report the size of all of the folders; however, this will not show you things your user account can not see. To find what is using up your disk space with Admin access and good visualization, I prefer the elegant UI of DaisyDisk which you can download as a demo to try. Or you can use free GrandPerspective.