My higher-level math courses were pure mathematics courses, and we pretty much always used Springer textbooks, which were only a few hundred pages long and the size of a normal paperback (i.e., not the size of, say, CLRS). When we didn't use Springer textbooks, we used other textbooks similar in size and length (e.g., ). I found these textbooks to be completely manageable to read as a student, and they were the best textbook-related learning experiences of my undergraduate years.
For subjects that are less dense, you need bigger books.
For instance all of the required and recommended books from the courses I took as part of getting a bachelor's degree in math from Caltech take less space on my bookshelf than the required books from just my first semester of law school.
Toss in the second semester of law school and include the non-required books I bought, and that would bring it to more shelf space than all the recommended and required books I bought for all the science and engineering classes I took in four years at Caltech.