We have a bug on the webopage which display the user's list account to a domain.
When we activate the option "Show mailbox size in users list?" in the virtualmin configuration panel, the quota display in the user list is twice the real quota used.
You can find an example with the screenshoot.
In user list the display mailbox size is 43.84 MB but in the user detail page the quota mailbox size is only 21.92MB and on the server the real quota is 21.92MB.
We have the last verison of webmin 1.620 and the last version of virtualmin 3.99gpl
The system is FreeBSD 9.0 but the bug is also on FreeBSD 7.x and 8.x