I agree this is tragic and under-reported, but I think you weaken your case when you exaggerate like this.. Its not a direct contest for "greatness" since they were so different.
As the other response notes, Tenochtitlan was possibly the largest city in the world at the time, and the Aztec empire contained many other cities with populations over 10k. The Inca built a massive empire (8-12M people) centered on the Andean mountains linked together by roads and suspension bridges and built monumental architecture using masonry techniques that are impressive even today. We're just starting to get a handle on the scale of the Mayan empire, because they built their cities in parts of the Guatemalan jungle we have a hard time getting through today - some of the more recent work has found evidence of raised causeway networks stretching over a hundred miles linking large scale settlements. Go further south, and the Amazon jungle basin has been considered uninhabitable because of the density of the jungle and the poor quality of the soil, except that we've found evidence of cultivation of plants and more recent evidence of large-scale settlements and waterworks.
The Spaniards did not walk into a backwards people or somehow just miss what they were looking at - the civilizations and cities they found were advanced and obviously developed and on a scale that, even outside Tenochtitlan, would have rivaled European cities at the time for both size and sophistication.
It's not that much of an exaggeration. Tenochtitlan--the capital of the Aztec Empire--is estimated to have been about 120-150k people at the time of contact, which is larger than any city in the Spanish Empire at the time. Even the Spaniards themselves, as they recorded in their journals, were astonished at the scale of Tenochtitlan.
this story is tragic and it is not widely known, and also..
the story of Jonah in the Hebrew bible describes a city of 100,000+ people .. and that was just one city somewhere at that time.. that was three thousand years ago in the Middle East
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.
I agree this is tragic and under-reported, but I think you weaken your case when you exaggerate like this.. Its not a direct contest for "greatness" since they were so different.