In the humanities and arts, I think there is more reading and the reading is more dense. Expectations in terms of quality of work may be higher. Most students learn how to NOT read everything assigned, and at the Ivy I know about, students organized their own study groups, and so on. The difficulty is balanced by stimulation and the cross-pollination possible in many college environments, private and public, but possibly more at top schools.
The top schools give 4 credits for each course, making 4 courses the norm for full-time. Publics I know give 3 credits for each course so 5 classes is the full-time norm. I was told that this is because the public school classes are less intense and burdensome, or cover less, or whatever, but the rigor of the education is certainly there with 5 classes per term.
I would not hold my child back from what he or she wanted, within the financial limits of our family. And financial aid is amazing at some top schools. I felt the way you do, about one of mine, but he made his choice and though he was occasionally stressed, he had a great experience.