Update: I just installed OS X Yosemite on both my Mac Mini (Server) and RMBP (Client). The replicated Server screen now has a 4:3 aspect ratio whereas previously it used to fill the 16:9 RMBP screen in correct proportion with no stretching. I have no idea what determines this behaviour. I didn’t do anything previously to produce the 16:9 screen, and have done nothing this time to produce the 4:3 screen. I don’t (and didn’t) see any options in either NoMachine or OS X that would affect this.
Now the torrent of messages no longer appear in the Console Log! Is this because I am now running Yosemite, or is it because my screen aspect ratio is now 4:3?