You make good observations about student ratings. The huge enrollments are in required courses for freshman and sophomore undergraduates. Tenured professors and professors in general tend not to like teaching those courses because they require a lot of organization and work relative to that required for a tiny upper level course. The student ratings of large, lower level “mass lecture” courses tend to average lower than those of the upper level small courses.
I'm not sure that's really true. At the very least it varies from school to school and department to department. The intro Biology course I took always has an enrollment of about 700 and the prof is *very* popular. Rating a little over 4 last time I looked, and lots of rave reviews on various internet forums, as well as RateMyProfessors. On the other hand, the Chemistry department at the same school has ratings in the cellar for virtually the entire department (chair had a 1-point-something rating last time I looked). Some departments (and probably some schools overall) have a habit of dumping lousy tenured professors they'd really prefer to get rid of into the intro courses that nobody wants to teach, and *that's* what generates the low ratings. I took a pre-Gen Chem course with a prof like this -- really a piece of work, obviously near retirement age, Ivy League PhD over 30 years earlier, pissed that he had to keep teaching this course (mainly for pre-nursing students, and non-science majors looking for an easy way to fulfill their science requirement), and was rude and patronizing to everyone. On the other hand, at this same school, the Psych department largely staffs its huge intro level courses with really *excellent* adjuncts, many of whom only teach there for a semester or two (I took Intro Psych with a guy who was doing the clinical year for his doctorate in clinical psych and moonlighting teaching this course -- he was really, really good, both in teaching/lecturing and in administrative management of the course).