Why did Baghdad became the largest city in the world?

It is the largest city in Iraq, not the world.

Baghdad (Arabic: بغداد‎ = Baġdād,) is the capital of Iraq and of Baghdad Governorate, with which it is coterminous. Having a municipal population estimated between 7 and 7.5 million, it is the largest city in Iraq, and the second largest city in the Arab World (after Cairo, Egypt)