L735
3.44/8GB Some of this is used by Telegram Messenger.
All other apps and data are stored on a 32GB card.
L930,storage sense 3.23 Cyan + P4D
L925 storage sense 3.16 Cyan + P4D
So looking at this almost 50% of the total Phone storage(for 8Gb Phones) is used by the system and remaining are used by the Here maps and some apps like WhatsApp, Music, OneDrive and Video which you can't install on SD card. So this all consumes almost full storage and then no space .
You can install maps on an SD card. I have all apps (and I download a lot) stored on phone memory, besides the HERE apps (and maps), games, and music and video apps such as Metrotube, MixRadio and Spotify, plus all photos and videos which are all stored on an SD card. Still got just over 1.5GB left on the phone, which isn't too bad. Certainly isn't a problem.
The HERE apps are stored in SD card but I can't find option to move already downloaded maps to the SD card.
Your System total is in Storage Sense list. "Lie back and think of England" but with a UK flag?