“I find little concern about Canadian immigration (legal or otherwise) by contrast.”
The reason for that would be that there is so little of either. Canada is a country of only 20 million.
In principle it doesn't. We could keep immigration quotas from other countries just as they are and still raise the quotas from Mexico. Personally, I don't really have much of a problem with raising overall total legal immigration quotas.
Even if we didn't, I'd say that Mexico and Canada are special cases just because they border the US. With most other places, you do have a chokepoint at points of entry into the US via plane or ship while with Mexico and Canada you have thousands of miles of border to guard and many places for border control to fail. Reasonably, we should have a different policy with respect to these different situations.
BTW the population of Canada is 33 million and Mexico is 108 million.