One way this comes about is when students avoid a professor. I am not sure what to do about this. Do you assign that teacher to a high enrollment course so he/she will be pulling her weight? Depending on the reasons for student flight this may not be the thing to do.
The other possibilities are that students are just not interested in a highly specialized course or the professor insists that his course must be capped at a relatively low number. For example suppose a professor is teaching international dance law (This is fictional with respect to my own school although am hoping someone will offer the course so I can audit it) and consistently draws 5 students. In his other course he draws a whopping 7 students. In fairness, when the school hired him it knew of these teaching specialties but it had no idea demand would be so low.
This is where the issue of Moneylaw comes in. What is the reaction of the professor? For some the reaction is "I am not pulling my weight. What else might I add to my repertoire of courses." For others the reaction is "This is what I teach and it's not my problem if the students to do not care." In short, it does not occur to this person that there is any obligation to take some additional responsibility. Let's call this professor "Minimum Bill" because when it comes to teaching Bill is out to do the minimum possible. (If Minimum Bill ever does get a high enrollment class you can bet he will go straight to the multiple choice machine graded test bank. Bill is also likely to argue for enrollment caps or that his 3 credit course should be listed as a 4 credit course for no reason other than it would mean that would take care of his teaching for the semester.) Why is Minimum Bill minimum? I am not sure. Over on classbias I would say it is because Bill has a sense of entitlement so often found among graduates of elite schools and, I am beginning the think, many pretenders. (Those with LLMs from those same schools and always identify themselves as "from" their LLM schools.) But, truthfully I do not know what accounts for this difference in attitude.
If I could rewrite the tenure and promotion rules, "pulling one's weight" would be near the top of the requirements. Please do not misunderstand, some small enrollment courses must be offered and some have to be capped. There is no reason that people teaching those courses could not volunteer to teach a larger enrollment courses. At my own school most professors, in fact, do just that.
I realize that "pulling one's weight" is not part of most tenure and promotion guidelines, at least officially. But scholarship is and, for me at least, the people who insist on their entitlement to very little student contact better have massive amounts of scholarship -- and I do not mean updating casebooks or treatises --or for me it is a no vote. Who needs them?