As Windows is not supported as NFS storage, this may be the problem, as the OS does not have the capability to keep the VMDKs thin provisioned.
Here is a good "workaround" for this kind of setup: http://miketrellosblog.arcadecab.com/2010/09/compressing-windows-nfs-share-to-simulate-esxi-thin-provisioning/
Hope this helps.