3.9 GB for maps seemed high, but that was nothing compared to my system files taking up over 24 GB of storage!! I'm in the process of restoring after a hard reset and will report back.
Nokia Lumia Icon
Lumia Black firmware
Preview for Developers 8.1
How many maps do you have installed? You could just install the maps that you need off-line the most?
Being you're using a Lumia you can install the Lumia Storage Check which will allow you to clear up some temporary files; http://www.windowsphone.com/en-us/store/app/lumia-storage-check-beta/c2adc981-83c2-4f19-8161-67149e4...