Time of settlement by Europeans has nothing to do with their population sizes. Canada’s territory is mostly tundra and artic forest. It cannot sustain large populations and serves little use, save for lumbering and the exploitation of other natural resources that might be found. Most of Canada’s population lives in the east of the country, where Ottawa, Montreal and Quebec are.

Now take the United States, where good farm land was so plentiful that the Federal Government started giving it away for free to anyone that would settle there. The eastern states were prime real estate when it came to farming. The climate is mild, the waters do not freeze over so seafaring trade and fishing are options. Then there’s all the natural harbours making those options even better. The Mississippi river basin is great for agriculture (breadbasket of America) and the enormous river system makes trade easy and transportation cheap. Really, the Eastern half of the contiguous United States is amazingly well suited to large scale human settlement and exploitation.

Concluding, we are talking vastly different hands being dealt here. Geographical size means nothing if most of your state is desert or tundra. Canada looks large on a map (especially with the Mercator projection), but no one is using that land for very good and fundamental reasons.