Redirected restores of individual documents from SharePoint GRT (Granular Restore Technology) backups are successful with zero bytes restored.
The same documents can be successfully restored when doing a restore back to the original location. The entire contents of the document is restored in this scenario.
This is working by design as per the following comments (taken from http://www.symantec.com/docs/HOWTO65637):
Note that only files redirected to disk will result in a usable item, Sites/Subsites/Lists/List Items redirected to disk will result in a 0 KB file with the name of the item restored but are not usable.