If Chicago had the same area as Houston, this would not even be a contest. Houston is 640 sq mi (land only) to Chicago's 227 sq mi (and literally 6% of the land area constitutes airports!). Cook County is 945 sq mi. If the city annexed enough of the county to get to Houston's land area size, and assuming it annexed the most dense townships, we would probably be about 4.5 million easily.
This is why city limits population does not matter. Houston proper might be a few hundred thousand short of Chicago, but the Chicago MSA is still millions more than the Houston MSA.
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"Eventually, I think Chicago will be the most beautiful great city left in the world." -Frank Lloyd Wright
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