> who was trading classified information for sex

That's really not how I read the scandal. A woman, who formerly had clearance for classified material, was engaged in an illicit affair. There is no evidence (that I'm aware of) that 1.) the classified information was ever leaked to someone without clearance, and 2.) that any actual trading occurred.

Clearance is not a blanket authorization to access classified information- only information you require access to for your position.

Lapsed clearance is even less applicable, obviously.

The scale of what Petraeus did vs what Snowden did is very different.

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.