You really can't compare units without knowing what the classes are. A unit is, generally speaking, an hour of class a week. Now, depending on the courses, that could be a large load, or it could be nothing at all. For example, if someone was to take organic chemistry (4), biochemisty (4), physics(4), along with physics lab(2) and O-chem lab(3) at my undergrad, then they would be at 17 units. That would not be an advisable course load. On the other hand, 17 units of political science (I have a minor in Poli Sci) can be be a very light course load.
The general guidelines is that 14-15 units (about 3 lecture courses and a lab or seminar course) is about what to shoot for if unsure of what load you want to take. 12 units, though, is the minimum to be considered a full time student.
On the other hand, specific programs might talk about units but give their students no, or very little say in what courses to take. For example, unlike undergrad, I had no say over the 16 units last semester or 15 of the 19 units (4 unit elective of our choice, can be split between 2 units this year and 2 units next year) this year.