This is probably more of an /r/AskScience or /r/AskGeography question but effectively you can look at the very fact that 90% of Canadians live within 100 miles of the US border. This is because of the climate of Canada being a lot colder than the climate of the US due to the higher latitudes. Similarly, Germany has a much larger population that the Nordic nations that aren't much further north in latitude.

I'm sure there are more historical reasons regarding immigration policies and larger population shifts due to diaspora taking in the American dream - but the root reason to the population difference is because most of Canada is uninhabitable for large populations.