You could painstakingly go through the census tracts to compile an exact number for Montreal's core, that being downtown Montreal and Le Plateau, and a few other areas (roughly speaking), but that's a lot of work. Just going by building type and the density of structures, it appears like Toronto and Vancouver have the larger cores (skyscraper wise and population wise). Inner city though, Montreal I think has the edge over Vancouver. Toronto clearly has the largest inner city.
There are a lot of what looks like 6 and 4 unit buildings in Montreal, such as seen here...
https://www.google.ca/maps/@45.4690741,-...ru_A!2e0!7i16384!8i8192?hl=en&authuser=0
That's on very large lots though, while Vancouver has 2 or 3 unit houses on smaller lots, so it might actually be close. That being said, Montreal has much better street life over a wider area, probably being a product of a city that developed before the height of the automobile age, and, well, it's just a bigger city.