The bigger schools like Harvard often have that for the first year classes, especially classes that are required for all or many majors. The class itself, and

The bigger schools like Harvard often have that for the first year classes, especially classes that are required for all or many majors. The class itself, and the lectures, will have many students -- but there's usually ample availability of grad student TAs for more individualized interaction. A grad student is usually perfectly capable of teaching a freshman class. Smaller schools and upper level classes generally just have fewer people. Edit: Keep in mind that teaching is something the professor usually just puts up with, to get the resources and credibility to do the thing they are really passionate about (research or writing).