The larger earth conductor would reduce the "touch" potential at the point of the fault compared to the reduced size, although as you said, that's really only likely to be an issue on a non-RCD circuit with allowable disconnect time of 5 seconds. (By the way, do the Irish rules have similar 0.4 and 5 second requirements?)
I know that American NM cable (Romex) used to have a reduced-size ground wire. I'm not sure when it was made full size or what the rationale behind the change was.
