While many Amerindians had impressive achievements considering their almost complete isolation from the rest of the world, your claim seems misleading.
> At its apex around 1100 AD, the city [Cahokia] covered about 6 square miles (16 km2), included about 120 earthworks in a wide range of sizes, shapes, and functions, and had a population of between 15,000 and 20,000 people.[5][a]
> During the 1st and 2nd centuries CE, the population of the city of Rome is conventionally estimated at one million inhabitants.
And there are much better examples according to some sources of large cities in the Americas than Cahokia.
> Hernan Cortés arrived in Tenochtitlan on 8 November 1519. Although there are no precise numbers, the city's population has been estimated at between 200,000 and 400,000 inhabitants,[27] placing Tenochtitlan among the largest cities in the world at that time.[28] Compared to the cities of Europe, only Paris, Venice and Constantinople might have rivaled it.
> In the 9th and 10th centuries, Constantinople had a population of between 500,000 and 800,000.[60]
> With the restoration of firm central government, the empire became fabulously wealthy. The population was rising (estimates for Constantinople in the 12th century vary from some 100,000 to 500,000), and towns and cities across the realm flourished.
> Kaifeng-Dongjing became largest and most prosperous city in China, with a population of over 400,000 living both inside and outside the city wall. The historian Jacques Gernet provides a lively picture of life in this period in his Daily Life in China on the Eve of the Mongol Invasion, 1250-1276, which often draws on Dongjing Meng Hua Lu, a nostalgic memoir of the city.[14]
> Around 750 AD, Chang'an was called a "million-man city" in Chinese records; most modern estimates put the population within the walls of the Tang city at around 800,000–1,000,000.[1]