Recover space on Virtual disks

I have mac OS X 10.3.4 , VPC 6.1.1, and several virtual disks and OS,
that are backed up, which is now much too big...

Then I have tried to use the option "recover space on virtual disks"
which is in the menu : window / virtual disk assistant

It isThey said there that the space has to be "reinitiliased" for the
assistant to work
And my question is now : what does "reinitialsed" actualy means?...

Because I have tried a lot around :
1/ First, I have speeded-disk my disk, with the "wipe" option
2/ I managed then that there is absolutely no more files at the end of
the disk (who knows, might be useful!!)
3/ Still without success, I used "eraser" to overwrtes 0 everywhere it
wasn't a file or a directory...

And still, out of 300 MB used, my disk keeps being 1.2 GB at the end
of the "recover space" tool... No way to reduce it

What can I do?

Big thanks