I didnt get your Question.
But lets see if this answers it:
When you create a Virtual HDD, the fedora files are installed on it.
This Virtual HDD file will be with some .vdi or something extention.
It will occupy the same amount of space that you might have specified when configuring the Virtual HDD.
Thus, VirtualBox uses your actual HDD to make a .vdi file which acts as your partition on your fedora 9.
I hope this is clear