Friend of mine has flown to Oz from the UK twice on Christmas Day's,once with Emirates and once with Malaysian. He says the back was full both times,"probably with people like him thinking it would be empty!" (his words). He said he was lucky enough to get an upgrade with Emirates cos the back was overbooked.
My daughter flew Oz to the UK with Malaysian on New Years Eve/Day. MEL to KUL was only half full but KUL to LHR was packed.
From the choices you have Cathay have the most leg room in economy.