Well, the pop-count is regarding the amount of people living in your lands under your rule. and if you have the brown-red land, there should be a lot of people.
Actually, if you want half of your people to live in the sea, you should probably take the bone-white lands. Anyways the brown-red lands are approx 36.000.000 km2, and the bone-white are approx 9.000.000 km2.