Which leaked documents would have warranted a harsher sentence?

Judging by the sensitivity of the documents is not how you judge the ethics of it. That by chance the damage from the leak wasn't a lot worse does not count in Manning's favor.

That doesn't make sense to me. Not every document is equally damaging. The punishment should fit the crime.

But the crime should be determined the action, not the chance outcome. The alternative is that the same exact action result in wildly different punishments due only to chance. This is unjust. People who have the same exact mental state and perform the same action should have the same punishment. If I punch someone in a fight and that person hits their head on the ground and dies, I should not get 10 years in prison simply due to the chance occurrence of him dying, while everyone else gets probation for getting in a fist fight where everyone walks away. The punishments should be exactly the same. The sentencing just should take into consideration the chance that someone dies and be applied evenly.