Are too many people in jail for minor pot possession?

I hear this repeated all the time, but I have my doubts.

Since almost all cases today involve plea deals, how can we be sure about this alleged problem? For example, let’s say a person is busted for five offenses, two of them violent, but the prosecutor makes a deal and he pleads to simple pot possession. Then the good-hearted liberal comes along and says, “why is this guy in jail for ten years just for having a little pot?”

The current system needs to be reformed, but we’re not going to fix it by making little tweaks without looking at the whole picture.