d4 said:yow, that's a lot of "empty" space!
i think it's worth noting (and this analysis bears it out) that especially in the medieval period, most nation-states would have only one really big city. Paris for France, London for England, Madrid and perhaps Barcelona for Spain, Istanbul for the Ottomans, etc. there simply were no other cities even close to their size in their respective countries, and this might have something to do with it: it took large swaths of the countryside to feed the city slickers!
Which, I think, in turn may explain why the DMG does not depict such really big cities. They would need to be so far apart (for those who care) that they would be effectively "off the map" for most camapaigns. Having more smaller cities allows the PCs to travel more and find the goods they need "close to the dungeon".