Well, compared to Montreal, Toronto's development patterns tended to enshrine single-family/semi-ownership; apartments were much more of an exotic "incremental" phenomenon, and often (as in Forest Hill) w/a bit of a secretarial/servant-class stigma. Yeah, Toronto The Good tended to be snottily un-foresighted about such things--though it's certainly ironic that the city went from pre-WWII apartment-phobe to Commie Block Central in the 60s and 70s...
I think that the story is more nuanced than what we are used to hearing about Toronto=home owners while Montreal = renters. We have to distinguish builtform from habitation patterns. While it was true that the primary residential builtform in pre WWII Toronto were houseform buildings, this does not mean that every house was single-family or even owner-occupied. There were boarding houses (precursors of rooming houses, and would have probably included meals), duplexes, triplexes, apartments above stores on every commercial street and most likely, rooms rented out in single-family homes.
The immigrant experience in Toronto post 1900, (as I can attest to from tales from my own family), would have seen multiple families or extended families within the typical Toronto Victorian house, a builtform whose flexibility made it do adaptable. Edwardian subdivisions like Palmerston Boulevard included small apartment buildings and purpose-built duplexes and triplexes cheek-by-jowl with mansions for the upper middle-class. Post WWI saw the growth of "Pullman-style" apartments all over downtown as large single-family homes (like on streets like Maitland, Charles, Isabella, etc), were replaced by two to three storey apartment buildings, without having to assemble a number of houses to make it happen.
Finally, the apartment buildings of the 40's and early 50's shown on this thread, reflected the changing demographics of the city post-war, both in terms of the increase in immigration, and the change in family structures such as seniors choosing to live on their own rather than with their children, the increase in "separated" households and most importantly, the increase in single-person and two-person households, a market which was not being addressed in the old houseform buildings. The huge growth of apartment neighborhoods in the 50's addressed this market and transformed the city and streets like St.George, Jamison and Jarvis. Even Rosedale was not immune to the post-war population pressures as the old mansion lots were replaced by small apartment buildings and many of the old houses themselves became rooming houses.
"Infill" apartment buildings of the 1950's:
St. George's Towers, St. George Street, 1957, Venchiarutti & Venchiarutti:
16 Rosedale Road, 1958, Bregman & Hamann:
Crescent Road, 1957:
Jameson Avenue 1950:
Today: