It wouldn't initially, but it probably would eventually. The majority of British immigrants during the 19th century went to the United States, not Canada. But if Canada had more land further south, more would go there than in OTL. Likewise with the German immigrants, more would end up going to Canada if Canada had a larger chunk of the southward arable land. There were a decent number of Germans who settled on the Canadian Great Plains, I don't think there's any political reason why less came to Canada, just land-related reasons.
I'm not sure about that. Did Germans want to be British subjects? Did the British/Canadian governments want large-scale German settlement? I do note that relatively few Germans immigrated to any of the British dominions during the 19th century, be it Canada, Australia, New Zealand or South Africa, whereas an absolutely huge number came to the United States, and a substantial number settled in parts of South America as well.
Also, a lot of settlers to Michigan and places further west were Americans, leaving the eastern states. They most likely wouldn't cross borders to settle in Canada (unless they hoped to spark a secessionist movement, à la Texas).
Canada's population in your hypothetical would be somewhat larger than today's but it wouldn't be 70 million (i.e., double its present size).
