Nitpick: The human birth canal does not limit the size of adult human brains. The human skull and brain both increase drastically in size from infancy to adulthood, and there is no upper limit to how big the ratio of adult-brain-size : baby-brain-size can get (and based on how quickly other large mammals grow in general compared to humans, I assume that the growth size of the brain could, in principle, be much faster than it is).
Other biological factors, including energy usage, and the mechanics of having such a large mass included at that position in the human, and others, do constrain adult human brain size.
Nitpick: The human birth canal does not limit the size of adult human brains. The human skull and brain both increase drastically in size from infancy to adulthood, and there is no upper limit to how big the ratio of adult-brain-size : baby-brain-size can get (and based on how quickly other large mammals grow in general compared to humans, I assume that the growth size of the brain could, in principle, be much faster than it is).
Other biological factors, including energy usage, and the mechanics of having such a large mass included at that position in the human, and others, do constrain adult human brain size.