[jagger27]

I think much of it has to do with how old the city is. It’s about as old as New York City but didn’t have the same level of explosive growth, which I suspect gave it time to settle in and grow more like a European city. The somewhat limited island space would have also contributed to denser growth.

[hclalpha]

A lot of it is actually, I think, a consequence of heating during winter. A lot of the denser parts of Montréal have been built at a time where Hydro-electricity wasn't developed. Houses were heated by coal and oil mainly, which was delivered in small streets build behind the building as the stuff was nasty. Getting greather density meant easier delivery. This is only speculation.