Why are Christian religions given extra colonists?

[Trin Tragula]

One idea might be to give extra colonists for Christian nations that get QFtNW only. Simply giving the nations with QFtNW extra colonists would increase the risk of getting a (very ahistorical) ming or japan that colonizes large parts of indonesia and australia.

Not really. As far as I know AI nations won't ever chose an idea not on their predefined nation list. QFTNW is not on the list of any of the Asian countries so they'd never pick that.
As for a human being able to pick QFTNW as an asian nation: I don't see why for instance China shouldn't be able to colonize if they chose to invest time and money into it. Their isolationism was by choice not because they couldn't explore. During the 15th century they did send out very impressive naval expeditions to India, Sri Lanka and East Africa.

 

[Trin Tragula]

That would require the game to keep track of population. It doesn't currently. Also the Netherlands managed to colonize quite a bit with not too much of a population (in fact if you add Denmark and Norways population in 1500 you'd end up with roughly the same population as the Netherlands according to what assessment of population I've been able to find) so a direct effect upon population size on colonization capabilities might not be that preferable.
Overpopulation in relation to a nation's size might be more relevant but is still not something the game keeps track of...