Canada only has so much arable land, which limits it's carry capacity.
I'll try and find sources, lol, this will probably take 40 minutes.
Canada has the 7th most arable land in the world, in the 21st century it hasn't hit anywhere near its population capacity let alone in the late 19th century.
It was never a matter of there not being enough arable land in Canada during that period, it was getting people over to settle it and work it, which wasn't the easiest with the US being such a magnet.
If as the OP suggests the US has some sort of stronger nativist period and cuts off immigration, Canada would rise as an option for the millions of Europeans still looking to leave. They wouldn't all go to Canada of course but more so than IOTL.