So they’re somewhat similar ages, give or take a century. Both big landmasses but one became the world’s biggest economy, famed for its cultural influences and the other became a small rather ineffectual and staid country. Why was that?

A lack of ambition?

Canada didn't get to bomb most of its competitors back to the Stone Age in WWII, and bankrupt the rest.

Also, the weather tends to put a damper on becoming the world's largest economy and cultural icon. Imagine what America would be like if 99% of the population lived within 100 miles of the Canadian border.

There's got to be more than that? The Americans basically got a similar start as a country, so why would one country have such a radically different economic and cultural trajectory?

I think it must be population and economy. The Eastern US (NY, Boston, Philly etc.) already had large scale much before Montreal or Toronto did. Investment went in and that opened up the rest of the country. Agriculture in the South was easier to develop than the great Canadian wheat fields. Essentially, the US gained critical mass a couple of centuries earlier. Then there was that independence thing.

Modesty.

This, ..... and that it's unspeakably cold for 4+ months of the year.

Having a large population helps the economy.

In 1900 the USA had more than double the population of Canada in 2016.

That's because people wanted to go to the US, why did they want to go to Canada?

But the growth of the population and the economy happened for a reason. It can't just be the weather.

In the last 1/4 of the 18th century, the U.S. became independent, and welcomed people from all over the world. Canada was still a British territory, mainly receiving immigrants from other Commonwealth countries.

So are you saying its the lack of economic development is because Canada only had immigrants from the UK and Ireland, Africa, Asia and Oceania as opposed to the immigrants that US received?

Does the geography of Northern Canada limit its economical growth and likely population, compared with more hospitable land mass of the USA?

The tenor of this thread suggests that Canada has in some way followed a path that makes it a less successful country than the US. This may be true... but ask yourselves 'Would I like to live in the US?' For myself, I wouldn't and thereby consider that Canada has followed a different and somewhat more satisfying route than the US.

Using that logic we should all want to go and live in Andorra.