Here's one study on the subject:
https://www.air.org/system/files/downloads/report/Delta-Cost...
The table on page 11 summarizes the story. The big consistent increases in spending are on student services and academic support.
The growth in spending on student services covers a lot of things. One big chunk is explained by the trend toward organizing and professionalizing things that were historically handled in a sort of ad-hoc manner by a student's academic advisor, sometimes by the Greek system, or by nothing at all: Career counseling and networking, academic advising, mental health services, stuff like that. Another big chunk is non-academic, non-instructional activities like student clubs and athletics. I believe advertising costs also tend to get lumped in under the "student services" budget item.
Academic support is stuff that's related, but maybe not quite so directly student facing: Dean's offices, instructional material development, the IT department, stuff like that.
