I doubt that backgrounds have anything to do with that. From my observations the system folder grows with OS/firmware updates. It also seems to contain some certain cached files that are somehow crucial for the system, thus aren't not stored in the usual temporary folder, where WP's very own storage sense could easily remove them. Any so called Cache Cleane apps from the store won't help you there either. These apps basically don't do anything besides filling your entire storage, then prompt you to reboot your device to let WP's very own cache clean mechanisms kick in.