Those big numbers come from simply taking the maximum possible sentence that can ever be given out for each charge, and adding them all up.
There are two things that make that unrealistic in most cases. First, the defendant is almost always charged with several similar or related crimes that have mostly the same elements. If convicted on more than one charge from such a group, they are only sentenced for one of the convictions.
Second, the sentence takes into account the severity of the particular acts that constitute the crime, and the prior criminal record of the defendant. To get the maximum possible sentence you'd need to have gone way beyond what ordinary violators of that particular law usually do, and you'd have to have a serious criminal history.
What Swartz was actually facing if he want to trial and was convicted was something ranging from probation to a few years, depending on just how much damage the court decided he caused.
If he took the plea the prosecutor was offering, he was facing up to 6 months.
Details with citations on the above are available at [1] and [2].
In the dozens of discussions of the Swartz case we've had in the last year here, the 35 year or 50 year myth has been repeatedly busted. Yet it keeps coming up in each new discussion--often from people who were in some of the previous discussions! Why is it so persistent?
[1] http://www.volokh.com/2013/01/14/aaron-swartz-charges/
[2] http://www.volokh.com/2013/01/16/the-criminal-charges-agains...