One quick fix I that worked for me (Iphone 5s) was completely deleting the podcast app. My system storage went from 7.88 GB to 4.2 GB, and curiously enough, I also cleared 7 GB of space. I was subscribed to multiple podcasts but not once did the storage page in settings tell me that the library was taking up so much space!
My phone storage screen looked very similar to "natarave"'s before I deleted the podcasts app.
I think Apple does not communicate well on how ICloud storage affects media usage. For example, when the Iphone Storage page calculates my photo memory, it will alternate between 180 MB and 3.5 GB used.
I am honestly disappointed that an issue so detrimental to user experience is so poorly addressed. Sometimes I can't take photos or download books when I want.
Good luck, Senior Ghiradelli