I think the National Archives should be able to tell you exactly what he took out of the files. Their job is to document things. They have a record of everything that was in the files.
Seems like a logical conclusion, doesn't it? If they won't, that is very suspect, and if they can't, that reveals glaring incompetence.
My suspicion is that the Bush administration, for reasons known only to them, swept the whole thing under the table and greased the wheels for Sandy's token punishment.
If the only thing he stole was a recipe for bunt cake, Bergler should have had his security status permanently revoked. The fact that he gets it back in a few years smells beyond bad.
And not to sound tin foil hat-ish, but I can find no logical way to explain away his light sentence other than the (deliberate) influence of this administration, in some fashion.