fdisk -l

excerpt from 'VirtualBox UserManual.PDF';

By default, VirtualBox uses its own container format for guest hard disks – Virtual Disk
Image (VDI) files.
The VDI files reside on the host system and are seen by the guest systems as hard
disks of a certain geometry. When creating an image, its size has to be specified, which
determines this fixed geometry. It is therefore not possible to change the size of the virtual hard disk later.

...

• If you create a fixed-size image of e.g. 10 GB, a VDI file of roughly the same
size will be created on your host system. Note that the creation of a fixed-size
image can take a long time depending on the size of the image and the write
performance of your hard disk.
• For more flexible storage management, use a dynamically expanding image.
This will initially be very small and not occupy any space for unused virtual disk
sectors, but the image file will grow every time a disk sector is written to for the
first time. While this format takes less space initially, the fact that VirtualBox
needs to constantly expand the image file consumes additional computing resources,
so until the disk has fully expanded, write operations are slower than
with fixed size disks. However, after a dynamic disk has fully expanded, the
performance penalty for read and write operations is negligible.
For either of the above two image types (that is, irrespective of whether an image
is fixed-size or dynamically expanding), you can also specify whether write operations
affect the image directly.