The wiki only glosses over the classified materials exchange in question with a single sentence.

Here's a better summary:

https://www.theguardian.com/us-news/2015/mar/03/david-petrae...

"The plea deal establishes that Petraeus shared “black books” with Broadwell with “classified information regarding the identities of covert officers, war strategy, intelligence capabilities and mechanisms, diplomatic discussions, quotes and deliberative discussions from high-level National Security Council meetings, and defendant David Howell Petraeus’s discussions with the President of the United States of America”.

There isn't a grade to oopsies with clearances and classified info. Either it's a breach or it isn't. I've read Petraeus's breach being considered legally worse than Snowden's, given what was disclosed.

To my prior point, prosecutors make prosecution decisions in regard to classified info selectively and politically. In addition to selectively meting punishment.