Taked from here => forums.virtualbox.org/viewtopic.php?p=41118#p41118
You could try something like this (see also Tutorial - All about VDIs: How can I resize the partitions inside my VDI?):
Copy contents from old to new disk. This will take a fair amount of time. (Here /dev/hdX is your original disk and /dev/hdY the new one).
dd if=/dev/hdX of=/dev/hdY
Warning: Make sure you do not mix up your input and output disks or you'll wipe all information from your original disk! (if= specifies the input and of= specifies the output.)
Once you've verified the larger VDI boots Windows fine (and disk size is as you'd expect) you can of course delete the old smaller VDI.
Edit: Instead of rebooting before you resize the partition you should be able to run partprobe and the hit CTRL+R in GParted instead.