Since latest Virtualmin version, the backup size reported in the confirmation email is very much exaggerated.
Before update 11.36 GB after update, 56.88 GB 
Thanks for pointing that out… that is indeed a bug. There’s a bug report regarding that here:
https://www.virtualmin.com/node/22130
It’s fixed up for the next Virtualmin version.
-Eric
Thanks, its indeed fixed in the latest release.