Debugging space usage discrepancies

You need to use CLI tools, like PowerShell:

sls "walker|retain" "C:\Program Files\Storj\Storage Node\storagenode.log" | sls "started|finished|failed|error"

or stop the node, rename the log, start the node back, a new logfile will be created and it will be pretty small.
If you prefer a dashboard, then you may setup a Prometheus storagenode exporter and configure Grafana as you like, include time when each of filewalkers was started/finished or failed: