This map, which comes courtesy of Statistics Canada, shows the evolution of Montreal’s population density since 1971. Basically, what you can see is that Montreal has become significantly less dense over the years. Between 1971 and 1991, high-density zones shrunk while the city sprawled outwards; since 1991, things have been more or less stable.
Some of this has to do with depopulation, especially in working-class neighbourhoods that fell on hard times in the 1970s and 80s. But most of it comes thanks to a decrease in household size; while a typical Plateau apartment would have been home to an entire extended family before the 1970s, only a professional couple or small family would live there now.
The same trend can be seen in Toronto, where population densities dropped in the immigrant-rich west end. Unlike Montreal, though, Toronto’s density decline stopped in the 1990s and actually began to reverse itself. Downtown Toronto is now more densely-populated than it has ever been, thanks mostly to a huge condo boom that has added tens of thousands of residents to the area. The same thing has occurred in Vancouver, which has only gotten denser and denser as the years have gone by.