Rome reached the million people mark in the 2nd century. All great cities of classical antiquity (familiar to the Spanish of that time) had over 100k pop: Carthage, Tyre, Byblos, Athens, Antioch, Alexandria, Constantinople...

Malthus was wrong on a ton of things, but his theory of population growth in pre-industrial cities effectively holds to the data (even according to economic historians).

Cities were effectively constrained by available food and transportation costs before the steam engine, so you'd get cycles of population growth, followed by population decline.

Particular places reached support capacity of surrounding land until something caused systemic failure, and population decline. Think of ancient China, for example with its cyclic population growth in a unified empire, followed by some natural catastrophe or political instability -> civil war as the political system weakened.