<p>Harvard has made a choice to have large introductory courses in popular fields, then use the professor time saved to offer a huge number and variety of smaller advanced courses on a wide variety of topics. Many of these are aimed at both undergrads and graduate students and I believe it is common for undergrads to take some such courses in their majors. </p>
<p>The alternative would be to divert more faculty resources to the intro courses, but reduce the richness of the advanced course offerings. From the department viewpoint this looks like a bad trade-off. Many of the students in the intro courses will never take another course in your department, so why impoverish the offerings to your majors and grad students for the benefit of those you will not see again?</p>
<p>The other alternative, extremely expensive and long term, would be to have a substantially larger faculty while holding student body constant. Then one could keep the teaching loads the same, cut the size of the intro courses, and retain all the advanced courses. But consider, to cut a 500 student course to ten 50 student courses, you have to use up 10 professor teaching slots. That adds up, even at Harvard.</p>