Qube Manager is a bit confusing about Disk Usage. Maybe it should be changed to just ignore the “root” volume for AppVMs: That volume only exists on disk while they are running, and because it’s a snapshot (of the TemplateVM’s “root” volume) it doesn’t really use all that much disk space.
You’ll get more meaningful results by looking at the individual volumes, e.g.
qvm-volume info debian-11:private
qvm-volume info debian-11:root
qvm-volume info A:private
qvm-volume info A:root
qvm-volume info B:private
qvm-volume info B:root