RE: set-frame-parameter

[Drew Adams]

> Perhaps you want (toggle-frame-maximized) instead of
> (toggle-frame-fullscreen)
> 
> As Michael pointed out, fullscreen things are ‘window-system’
> things. Whereas a maximised window can be resized.

Whichever is preventing frame-size changes.
That's the point.  If a user can work around
this inconvenience with an extra sexp then
Emacs itself could presumably make that
workaround unnecessary by doing it automatically.