do you think every province had a million people in 1444? Most provinces dont even have a million people now
No. But the worlds population in 1800 (the date for the above picture) was about a billion. I believe there are a bit over 1000 provinces. Therefore, almost a million rebels per province in order to have rebels equal to population.
If we instead look at the game start, population was about 400-500 million. However, there are also far less provinces for rebels to spawn since there will be so many un-colonised provinces.
Evidently not every province has a million people, most will have far less. But there will also be provinces with more than a million people, where larger stacks of rebels can form.