This makes no sense. Every large city is dark green (the best.) Vancouver, Calgary, Edmonton, Toronto, Ottawa, Quebec City. Even Montreal is light green. Most smaller cities—like Halifax, Winnipeg, Saskatoon—are also dark green.
A city is defined as 100,000 population or greater in Canada, none of the cities you mentioned are small given that context. A good example of a small Canadian city is Thunder Bay. I'd give you more... but I'm from the GTA. ;)
You're right, there probably are other factors in play here. But there is almost certainly not one reason to explain everything. Oil looks quite likely to be a major reason, but we should find out what the other reasons are.
