The private schools with gigantic endowments can afford to “waste” space in order to provide a more relaxed change-of-major experience and handle short term increases in a major’s popularity. Less well endowed private schools (e.g. CMU with respect to CS) and many of the popular public schools face financial and political pressure to use all available capacity, and may not have the budget to easily add more (or they have to have internal political fights to take funding from an under-enrolled department or other part of the school to expand the capacity at the “full” department).