"The --resize option allows you
to expand the capacity of an existing image; this increases thelogical size of a
virtual disk without affecting the physical size much.[32] This currently works
only for the VDI and VHD formats, and only for the dynamically expanding
variants. For example, if you originally created a 10G disk which is now full,
you can use the --resize command to add more space to the virtual disk without
having to create a new image and copy all data from within a virtual
machine."