Is this a bug
What protocol do you use to connect to your NAS box? SMB? NFS? If NAS unit does not properly support updated versions Windows has clients to you'll get mismatched volume sizes, wrong dates/attributes and possibly scrambled file content. For SMB you may wish to play with version settings, see this link:
http://www.petri.co.il/how-to-disable-smb-2-on-windows-vista-or-server-2008.htm
For NFS you need to do similar things (just assume you use SMB by default as more common).
I am experiencing a similar issue. Where Win8.1 reports a ridiculous size for files on the NAS server.
For example on Win7 a folder is listed as 8.8MB on Win8 the same folder is listed as 6.75GB. Attached is a screenshot of the folder listing comparing win7 and win8. http://social.technet.microsoft.com/Forums/getfile/400183