The major change is the population. Istanbul’s population over the last 2000 years has always been between 200.000 - 600.000. Istanbul was the crowdest city on earth in the 15th Century.
The major population rise began in 1950’s. The population was then 1.000.000 the first time in history. But after that the population rose dramatically and in 2019 the estimated population is very near to 20.000.000 people. ( Official 18 mil. )
Because almost 40% of trade, services and manufacture of Turkey is in Istanbul, it still is an attraction to domestic migration.