After some discussion in the 'What is up with the victory points thread?' I believe it is time for a separate thread on this issue, here are some starters:
I would say provinces that had the most inhabitants should be displayed by manpower (Indonesia is an extreme example of this, apparently 0 people lived there).
On the Manpower thing: IMHO manpower does not reflect absolute population, but population that can be readily drafted into the army. If the province has lots of population, but they're all subsistence farmers, it should have zero MP (unless you have another agricultural region with overproduction you can feed them with). The actual numbers on the map are probably just as messed up as VP and IC are, but low MP in populous underdeveloped counties is somewhat justified. It should be handled by technology and infra only, but the engine is too limited to do it that way.
MP values are delicate to twink : it's dangerous balance-wise and I agree with the philosophy that MP is "mobilisable manpower to fight for their current country in 1936 after colonial multipliers", neither strict headcount nor "MP in 2013" nor "strategic value in 2013".
@manpower diskussion: At the moment Indonesia has no manpower if released. But it has a drain on manpower due to aging of manpower in province buildings. Imo any country controlling all its national provinces should have a daily increase in manpower, even if it is a very small one. Right now it is somehow messed up.