Auhm -- when starting to write the bug report I decided to make sure I got all the facts right.
Unfortunately, it turned out that I had run an old Emacs 23 on X11 -- which do run the window-size-change-functions. When testing on an Emacs 25 on X11, it turns out that it doesn't. Back on the mac, I see the same pattern.
In other words: On Emacs 23, window-size-change-functions is called when the user manually resized the frame. On Emacs 24 and Emacs 25 it isn't.
Sorry for the mixup.
Anyway, I just reported this regression as bug#21975.
I noticed this since Follow-mode no longer aligns its windows automatically when the user resized the frame. In practice, this is not a big problem since they will become aligned at the first user interaction instead.
-- Anders