You were the one who included Geelong as a club but ignored the population though, Geelong is a city in it's own right, Melbourne will overtake Sydney by 2026, and the only city growing faster than Melbourne is actually Geelong.
But since you asked
Greater Sydney's population is already inflated by the central coast's, whereas Greater Melbourne does not include Geelong, if this wasn't the case Melbourne would already be Australia's largest city
Greater Sydney (Greater Capital City Statistical Area), as classified by the Australian Bureau of Statistics, extends from Wyong and Gosford in the north to the Royal National Park in the south and follows the coastline in between. Towards the west, the region includes the Blue Mountains, Wollondilly and Hawkesbury. Greater Sydney covers 12,367.7 sq km and is made up of 35 local councils.