Australia is often used by demographers as an example of a highly urbanized country, but the contrast between Canada and the U.S. is perhaps more interesting, because we're on the same continent. And indeed, looking at a biggest cities comparison list of both countries, it's striking: of the 3 biggest cities in North America, 2 are in Canada. But as you include more cities of decreasing size, the % of Canadian cities in the rank list converges toward 0.1. (Y axis is cumulative % of Canadian cities in the list of biggest cities in North America, of cities with at least 100,000 inhabitants.) Canada's smaller population is enriched for bigger cities.
Even better than the rank list is the difference in urbanization, i.e. the percent population in cities by size. And there it's even more striking: Canada's biggest city alone contains 16.2% of the country's population. To get to that level of % urbanized population in the U.S. you have to add up the first 60 cities.Y axis is % of population in cities, counting cities of decreasing size up to that point (again, as far down as cities of population 100,000).
Climate? Less of a homesteading policy in Canadian history? Less aggressive property purchase incentives in the automobile era? Or Canadian agriculture taking off only post-mechanization (and hence never requiring settlement by large numbers of people in the first place) since the climate is harsher? The thing to do to start distinguishing between these alternatives would be to generate a 3D surface of this last plot with historical data to see if there are any obvious inflection points. And with the last possibility, the question is whether the prairie provinces have areas losing population in the same way that the post-mechanization Midwest has in the U.S. (More on agriculture and settlement of central North America here.)