Archeology is my fav.
There is strong evidence it was a multi-ethnic city, especially since there are distinct ethnic neighborhoods based on artifacts such as pottery. No trace of writing or how the city and government were organized, and whether a ruling elite called the shots or if there were ruling families from different ethnic groups working together.
(Seriously, though, _is_ anything much known about them beyond that?)
...so what? Why would you compare "the size of Teotihuacan" to "the area enclosed in the Aurelian Walls"? Why not compare it to "the size of Rome"?
I can think of one reason you'd do this...