Earlier quoted context omitted.
You're assuming that teaching ability scales without scaling the workforce. Having an instructor provide for more students means each student gets less instructor-time. More instructor-time increases the learning ability of most students. And so class size is an important metric. For most universities, their biggest product is their own marketing and promotional material. Now that they are corporatizing, they are pai…
I'm thinking that 'instructor-time' is a myth. A class of 20, say 3 lectures of an hour a week - that's just 9 minutes per student if they spent ALL the time interacting. But they don't; more like a minute per student tops. No, instructor-time is moot. Maybe instructor training is important; maybe a quality syllabus helps; a good book is key. But the instructor/lecturer can lecture 10 or 100 equally well.
-Prepare lessons
-Answer student questions (during and) after class
-Grade homework
-Prepare, proctor, and grade examinations
And instructors rarely spend their entire instructional period interacting with students; usually, they have a lecture they deliver for the majority, and a small time at the end for questions - about the material, class errata, unrelated questions, or even just to bask in the glow of someone with expertise in the field.I'd estimate that 1/4 to 1/2 of students will stay after for question time, or show up to office hours. Those few minutes are wildly different for a class of 10 vs a class of 100.
If you have a 50-minute class, and you choose to have a 45-minute lecture and 5 minutes of question time, you can spend 1 minute each with the 1/2 of your 10 students. With a class of 100, you can have a crowd of 50 where maybe 10% of them can get your attention.
Additionally, many lecturers have office hours - time specifically allotted to meet with students. If you kept 2 hours a week, you could give an additional 12 minutes to each of your 10 students a week. With a class of 100, you can hope that only 1/4 of your students show up.
Furthermore, in a lecture of 10, it's very easy to ask an incidental question and go onto a short educational tangent. In classes exceeding 25, it's very difficult to retain control of such a large body of people, and so the instructors work very hard to ignore such tangents and keep everyone on track.
Not to mention the order of magnitude difference in grading 100 assignments vs grading 10.
I'm less-convinced that instructor-time is a myth. But maybe I'm biased by experience.