Does economy of scale apply to universities?
Yes, economy of scale does apply to universities.  Having more students reduces the operating cost per student.
The universities you have listed are research universities.  They prioritise spending on research, and not students.  They do not care about revenue as they have plenty of money and do not exist for the purpose of gaining more money.
For institutions that have little money and get most of their money from students, I've sometimes heard it said that 300 students is &quot;enough&quot; to achieve economy of scale.
Some universities achieve economy of scale using purchasing consortia.  In these constoria, several universities work together to negotiate a favorable contract from a vendor.  This is quite common for public universities.