Yeah, agreed! Snowden gave reporters a lot of information which was then released to the American people on the quite probably illegal activities of their "security" apparatus.

Petraeus had an affair with a reporter.

One of those things made it possible for Americans to know what their government was/is doing with great personal risk to the whistleblower. The other....didn't.

I don't think scale is written into the law.

While I can't speak to whether or not it is written into the law, there is historical precedent on the matter. Look up Clayton Lonetree for one where the scale was low. He was given a shorter sentence than is typical of espionage. Look up Aldrich Ames or Robert Hanssen for someone where the damage was estimated to be much greater.