CIFS vs. DF vs. VMWare Reported Space Figures Mismatch

Hi All,

I've got some volumes on a FAS3040 (running 7.2.4P9) being accessed from a VMWare ESX box via NFS for the storage of virtualized server images.  If I look at the volume properties using the DF command in ONTAP it shows the following for example:

   Filesystem                    total     used     avail     capacity  Mounted on
   /vol/virtual1a/               675GB   186GB   488GB   28%       /vol/virtual1a/
   /vol/virtual1a/.snapshot  450GB   191GB   258GB   43%       /vol/virtual1a/.snapshot

However, if I connect to the same volume using CIFS, and in Windows Explorer select all folders containing the VMWare image files, do a right-click and select properties, it shows me the following (snapshots are not turned on for viewing)

   Size:             304 GB (326,706,623,377 bytes)
   Size on disk:   304 GB (326,709,755,904 bytes)

Interestingly, Virtual Centre displays the same figures for used/free/total as ONTAP does.

Has anyone else come across this apparent mismatch?  Any ideas what causes it?  We've been a CIFS only house up to now, and are only starting to dabble with NFS for the storage for VMWare, so my experience with NFS is minimal.

Is it (a) something to do with the connection to the volume, i.e. VC connecting via NFS, ONTAP looking internally, but Windows connecting via CIFS? (b) Windows seeing a 30GB virtual server as taking up a full 30GB whether the "internal" drives are full or not, but ONTAP runing at the block level only counting space used and not counting empty space in the virtual server's internal drives, or (c) something completely different?

Thought I'd try here first before raising a case with NetApp.
Thanks for reading.