I think that is an excellent question.
Certainly the federal system in Canada and in several other federal countries is an important aspect to take into consideration. But I think that proportional representation is compatible with both federal systems like Canada and unitary systems like Sweden, Norway, Denmark, and the Netherlands. I think the main thing to consider is to look at other federal countries that do use proportional representation. Germany is, of course, a federal system; it's not as large geographically as Canada, but it has a much larger population than Canada, and it uses proportional representation. Switzerland is a federal country and uses proportional representation. Austria is a federal country and uses proportional representation. And I've mentioned Australia already as an example of a country that's both vast and does not have a very large population—it's much less than Canada. It uses proportional representations for senate elections.
So I think these two factors really can be considered separately and I see no reason why a federal country cannot do well with PR.