Compacting Parallels Virtual Hard Disk Files

Can someone explain to me how Parallels calculates virtual hard disk sizes displayed in VM Properties?

• When I initially created this VM, I created a 10 GB expandable virtual disk image to run the host operating system (Fedora Core 5 Linux).
  
• The Mac OS X Finder reports that the disk image HDD file is 7.42 GB in size.
  
• The host operating system (FC5 Linux) reports that 6.8 GB is actually in use on the disk.
  
• Yet Parallels, under the VM Properties > Hard Disk > Advanced panel, under the Optimization heading, says:

Actual size of disk image file is 3497 MB. Click Compact to reduce the size of the hard disk image file.​

This makes no sense to me. How does Parallels arrive at the number 3497?

Also, when I click the Compact button, a progress dialog is displayed for quite some time, indicating that some sort of progress must be happening, but after which the size of the file is unchanged.

I even went so far as to run ImageTool and Defragment expanding virtual hard disk before compacting - same results! The HDD file size is unchanged.

 

i've wondered the same thing. as well as how to expand the virtual hard disk if i need to. apparently merely changing it's size is not sufficient.....