My Lumia 820 already running low of space when I download large maps. I have to delete some games and apps. 8GB is too low for a smartphone nowadays. But it should not be a problem had Microsoft/Nokia allowed maps to be installed on SD card.
Have you tried to use skydrive for more space?
SkyDrive kinda defeats the purpose of offline maps, doesn't it?
My Lumia 820 already running low of space when I download large maps.