Previous comments have not successfully captured the true magnitude of New York.
In terms of subways, imagine 5 subway tunnels containing 4 tracks each, all running within 500m of Yonge on either side. These subway tunnels would eventually branch off into 17 unique lines farther out in the suburbs. Commuter rail would include two hubs each larger than Union Station, and all trackage would run underneath perhaps a vastly more developed University rather than in a dedicated rail corridor. For roads, there would be about 12 one way avenues each 4-7 lanes wide running parallel between Bathurst and Jarvis.
Previous comments about density are way underestimated as well. A density similar to St. Jamestown, though with smaller but more compact buildings, would exist all the way from the lakeshore to about Steeles, between Bathurst and Mt. Pleasant, save a narrow sliver for Central Park. There would be two CBDs each many multiples larger than our own, with one centred on Front and the second between St. Clair and Eglinton. The remainder of Toronto proper is similar in area to the other 4 boroughs. Most of that land would carry an average density greater than the old City of Toronto, though an area corresponding to perhaps Scarborough (representing Staten Island) would be quite suburban in nature. The City of Toronto would include over 400 subway stations in its borders.
When all is said and done, about 8 million people would live in the area currently housing 2.5 million. The rest of the developed GTA would double or triple in size, and house another 12 million people.