Try creating a new pst you will see this zero size pst is non-zero in Windows Explorer.

More info here http://www.techtalkz.com/microsoft-o...-mismatch.html "The file sizes reported by outlook do not include hidden messages - published forms, rules, views etc so it will always be smaller. There is other overhead that outlook doesn't report but I don't recall the typical difference between outlook and the file system's reported sized."

You can try compacting the pst. The size in Windows Explorer will probably decrease but remain more than shown in Outlook.