That's just not true. Many costs scale with the number of students. Dorms, food, academic buildings, administrative workers, faculty (no you can't teach every class in batches of 500 like Econ 101, even then there is an army of graders), software licenses, land underneath the buildings, utilities and infrastructure, roads and parking, IT infrastructure and people to run it, grounds maintenance, student healthcare, etc. You can't just keep packing an arbitrary number of students into the same classroom.