In 1914 the population of Istanbul was 909,978
Muslims 560,434
Greeks 205,762
Armenians 84,093
Others 56,689
In European Turkey the total number of Greeks was 309,657 in 1919, also there were 118,000 Armenians and 45,000 Jews.
So there would probably be a population exchange. In OTL 1.5 million Greeks left Turkey and another 500,000 Muslims left Thrace during 1919-1924.
This is in the assumption that everything remains like otl until 1914.
Having a higher initial population doesn't guarantee control. The Turks after all began as minority in Greece in Anatolia and in Constantinople.