CX139/8 is operated by HK based flight crew, other SYD flights are operated by Aus based crew all of whom are Airbus rated.
Someone also mentioned the increased connection capacity, both from early morning Europe/USA arrivals to SYD and for the morning wave of CX/KA flights after arriving in HK at 5am.
Mostly I think it has to do with the increased belly cargo capacity of a 777 over the 330 as CX may be looking to cut the twice weekly 748F over the Aus summer period.