In recent years Canada has experienced absurdly rapid growth in total population by any standard. It doesn't matter that much of it is from immigration, that's beside the point. Even without any immigrants the population would've been growing still. It's already grown to levels that stretch our natural resources per capita much thinner than they used to be. I remember even in my youth, in the 1980s, my father pointing at the vast new suburbs surrounding his hometown and lamenting how much of the best farmland had already been paved over. Growth like this cannot continue, it's got to stop at some point. Slowing it down a little would be nice, if we have that power. It wouldn't magically fix all our problems with housing and everything else, but it sure wouldn't hurt. It would at least buy us time to learn how to do development in less environmentally destructive ways.
On the other hand, borders suck and I'd prefer to live in a world without any. It's a dilemma keenly felt and little understood even in Canada where it's endlessly discussed.