Why did he get the mazimum? He had committed a crime, yes, but it still was for an ally, not an enemy, and other spies who have been for enemies have gotten a considerable amount smaller of a sentence.
Please don't interpret it as criticism, but this topic has really been beaten to death on this board. I won't presume to speak for others, but my view is that (1) he knew absolutely what the penalties for releasing that information were when he signed the releases that are mandatory for that sort of classified access; (2) it is immaterial whether he gave it to an "ally" or to his mother or to Vladimir Putin; (3) his sentencing wasn't done with respect to other cases but strictly to his own. He got the maximum because it was the maximum. What others got they got for reasons specific to their separate cases. There is no mandate for "fairness" in sentencing.
I don't speak entirely in a vacuum - for a time I did a similar job with similar access. There is no excuse, there is no justification, and he can expect absolutely no slack from me. Your own opinion, I am sure, may vary.