You're leaving out most of what the instructor does:

  -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.

It depends on the instructor and the way the university is set up. I had classes as an undergrad which the professor had literally been teaching for decades - these guys hadn't spent time on a lesson plan since before the Beatles broke up. And they had grad students to grade papers and tests, plus teaching assistants the students were expected to see first before bothering the professor.

The test questions were just mixtures of questions from previous years.

So three hours of class, plus two (mandated) office hours. I'm assuming there was some time spent coordinating the grad students, but still... that doesn't seem like much work.

You had one professor who had a system worked out. How did the rest of your professors handle their workload? I too had a few professors that had it all figured out, a lot that were hard-working professionals, and a few that struggled to be on time to class.

Much like anywhere in life.

Not one professor. Most of them for the first two years. The exceptions were younger guys who hadn't taught the class before, and there weren't many.

> But the instructor/lecturer can lecture 10 or 100 equally well.

Depends on the class. I once had a colleague say that he'd teach the intro class to 5000 students in the basketball arena. For that class, at that university, it would probably work.

I'm teaching a senior-level econometrics class to 10 students right now. Everyone has a laptop in front of them, typing into RStudio throughout the class. I help students when they can't get things to run properly. We have plenty of other interaction. They each do unique homework sets and their own project. There's no way that class would work with 100 students, or for that matter, more than 15 students.

I thought roughly the same thing until this semester, my fourth in university. I took my first class with under 10 people, a combinatorics class, with an outstanding teacher. The class was demanding, and it is the first time I have attended office hours on a weekly bases. I learned more than I ever have in any single class in my academic career, and I can directly contribute that to the amount of interaction I had with my professor. Now, had he been less adept at explaining concepts, perhaps it wouldn't have made a difference. But having easy access to him directly impacted the amount I learned.