It's not that simple. A huge part of what makes a particular college attractive is selectivity. Acceptance rates are tracked and reported. Harvard and Yale are

It's not that simple. A huge part of what makes a particular college attractive is selectivity. Acceptance rates are tracked and reported. Harvard and Yale are ranked so highly that they could get away with it. But they won't do it for other reasons. Mainly that they don't need the money coming in from tuition. If you take a private school a bit further down the list though, their rankings would tank if they accepted twice the students. Also there are other limiting factors. Many colleges don't have the physical space to just double in size. Their isn't land for sale within walking distance of their current campuses. Adding satellite campuses is one way around that, but that adds logistical problems that will reduce desirability. In addition to satellite campuses not always inheriting the reputation of the main campus. Additionally you can't simply double the student to faculty ratio b/c that would tank rankings, and hiring double the faculty takes time. You also don't know which departments you need to scale beforehand. If you decide to double your enrollment in the next 5 years, which majors will the new students choose? It's unlikely the ratios would stay the same. Good programs don't always scale well either. If a department worked hard over the last 20 years to build a good reputation, they can't just hire 20 new faculty members and teach twice the number of students without impacting quality. Schools do expand all the time, but it is a very slow process that by its nature can't keep up with a constant growth in demand.