cockpit-machines currently uses block.<num>.allocation to show in-use disk space as saw by the host. However this seems wrong, as the right value to be reported should be block.<num>.physical. From virsh man page:

block.<num>.allocation - offset of highest written sector in bytes
block.<num>.physical - physical size of source file in bytes

When relying on allocation, a wrong value for USED is shown by cockpit-machines. For example:

block.1.allocation=3222331392
block.1.capacity=214748364800
block.1.physical=121545744896

Note that for stopped machines, allocation and physical matches - so the issue is apparent on powered-on vm only.

Upstream bug report at https://github.com/cockpit-project/cockpit/issues/9052