This is how their urban areas changed from 1950 to 2010.
Chicago has considerably more low density suburbia. About 30% of the urban area lives in <3,000 ppsm (1200 p/km2) census tracts, compared to less than 5% for Toronto. In Toronto, most of these very low density census tracts contain particular large non-residential areas (parks, industrial, etc). The typical Chicago suburbia is comparable to Toronto's lower density suburbs, while the average Toronto suburb is more comparable to the lower density half of Chicago's pre-1950 development. Brampton, Mississauga, North York, Scarborough and Etobicoke all have weighted densities above 10,000 ppsm (3800 p/km2), are predominantly post-WWII and make up over half of Toronto's urban area population.
Area: 450 sq mi (1,164 km2)
Standard Density: 10,149 ppsm (3,918 p/km2)
Weighted Density: 27,099 ppsm (10,463 p/km2)
Population over 20,000 ppsm: 2,593,494 (7,772 p/km2)
Population over 10,000 ppsm: 3,520,152 (3,861 p/km2)
Urban Area: 112 sq mi (289 km2)
Standard Density: 9,742 ppsm (3,761 p/km2)
Weighted Density: 21,337 ppsm (8,238 p/km2)
Population over 20,000 ppsm: 585,667 (7,772 p/km2)
Population over 10,000 ppsm: 842,973 (3,861 p/km2)
Urban Area: 3,458 sq mi (8,956 km2)
Standard Density*: 2,587 ppsm (999 p/km2)
Weighted Density: 9,047 ppsm (3,493 p/km2)
Population over 20,000 ppsm: 1,120,257 (7,772 p/km2)
Population over 10,000 ppsm: 2,594,751 (3,861 p/km2)
Urban Area: 911 sq mi (2,360 km2)
Standard Density*: 5,684 ppsm (2,195 p/km2)
Weighted Density: 14,853 ppsm (5,735 p/km2)
Population over 20,000 ppsm: 993,659 (7,772 p/km2)
Population over 10,000 ppsm: 2,871,565 (3,861 p/km2)
*Urban areas are defined by dissemination area for Canada and I think census block (or block group) in the US rather than census tracts. These numbers are based on all census tracts that are at least partially contained within the urban area. So some of them will include non-urban land, bringing down the density compared to the numbers you might be used to.