I'm sure it could have been a longer sentence for him, especially if the prosecution say they were pushing for 60 years. The US does have a parole and good behaviour system, so I would imagine he won't actually serve 35 years anyway.
It is a tough one with him though. The US Government needs to send a strong message to stop people doing similar and make it clear that they can't have their secrets known. On the other hand though, he bought some information of worldwide public interest that showed how they have effectively covered up and allowed the deaths of innocent people.