~6.954 (as of 2016 so actually higher) million of those 15m people live in 8,244 km^2 in the Greater Toronto Hamilton area, which works out to 843 people per square kilometer (vs 3368 for Istanbul).
Yes, 1/4 the density, yes, but nowhere close to the bizarre comparison you gave, where you used literally uninhabitable/barely-settled areas of the province (Ontario is huge and mostly rock and lakes) and compared to one of the densest cities in the world. At least compare urban area to urban area.
Toronto is the 3rd or 4th most populated city in North America, depending on how you count. Not some quaint arctic getaway. Also sits at the same latitude as e.g. Marseille in the south of France, or Florence in Italy. So not exactly northern at all.
Also if we restrict to just the actual technical city of Toronto proper, the density is higher than what you gave for Istanbul: 2,794,356 people in 630.20km^2 == 4435 people per square kilometer.
So... Check your biases at the door.