* Wikipedia numbers *
Chicagoland - 28 120 km squared
Population - 9.6 million
GTA - 7 123 km squared (only 1/4 the size)
Population - 6.2 million (2/3 the population)
So, the GTA is more dense, but the greater Chicago area is bigger than the GTA in both size and population. It's still a good place to begin comparisons. I see no reason why the 905 shouldn't be included rather than excluded. It isn't Toronto's fault that Chicago doesn't build many high-rises beyond its downtown core.
In the GTA, those endless km's of sprawl are filled with tall buildings, in Chicagoland those endless miles of suburbia are filled with low-rises.