Maybe it's just my university, but I spend 21 hours a week in class, even though I am officially taking 15 "units"). Most STEM classes are a 6 hour/week time commitment even though they are only listed as 4 units officially.

Also, class time is split about 60/40 between professors and graduate (sometimes undergraduate) student instructors. Class size for the student instructors is 25-30 and class size for the professors is anywhere from 100-1400, depending on the class. Some upper div classes are notably smaller, depending on the department. It's my understanding that this setup is pretty common at large public universities.