File systems tend not to have large data structures. They are relatively modest data structures which manage bulk access to large blocks of data. A subtle distinction, but important in this context.

Really? Even huge volumes with loads of tiny files? Mail and Usenet servers? Build servers? What is your definition of large and modest here?

The data structures under discussion here are those used to track individual files. Yes, there can be squillions of them, but as data structures, they are, in fact, rather modest.