Devvy
Donor
This is a good reference map for this topic I think.
Nice map. Much of the lower density areas are mountain areas though, so difficult to build on (apart from the chronic earthquake risk!). Difficult terrain + earthquake risk = expensive. There's a reason you already have multiple generations of a family living in one house; the Japanese housing market is already too expensive and apartment sizes are already tiny. If people can't afford a house now, saying you're going to build more of the expensive homes if not going to provide actual accommodation for people.
Just to be clear; you could definitely have some extra population. You could probably squeeze in some decent extra population on Hokkaido, and each city could be a little bigger, but I just can't see there being anywhere near enough space for a doubling of the population. High end plausibility-stretching estimate; maybe 150 million instead of the OTL 125 million'ish.