Not a problem in other sparsely populated, geographically large countries, like Australia.
Take british columbia, a vastly big place by US standards. Drop one 50-mile circle on vancouver and you have 75% of BC's population covered in one service area. If you draw a line from the ski hills north of vancouver, to alaska, you will cross only two roads. Everything to the north is empty by us standards.
In addition, Cell phone networks need to cover the area people travel, so unless Canadians only travel in an area 11% that of those in the US, the point stands.
Honestly, that's probably not far off. Canada is enormous, but the vast majority of it is completely uninhabited or very, very sparsely populated.
There are large sections of the trans-canada highway that aren't covered (the only east west highway, and the main road across the country). I can't really count on having cell service on the highways between towns when I'm travelling in Canada so they aren't really providing much coverage there.
Just to drive the point home here's a map of Rogers coverage. They don't have a single tower in the three provinces that make up the northern half of the country. Canada, despite being large is really a VERY centralized country. 90% of Canadians live within 100 miles of the US border.