Where does IOS Store Offline Music? (NOT for the reason everyone asks to steal music)

[Cheapo]

I know this question has been asked before, but I'm not asking for the reaons of trying to copy it or what ever. Let me explain where this question came from. 

So a few months ago my device would ping me when I got home and say "Hey, your icloud is full, buy more storage or delete stuff blah blah blah" So a few days ago I finally got around to looking at this. I have like 900 pictures and maybe 20 minutes worth of video, so I thought it may be reasonable that was causing it. My deviced said it had 5.1 GB of pics on the camera roll. So I backed up my pics and vidoes, and deleted about 85% of them off (wasnt sure if videos were backed up on icloud, got rid of them just in case). So after leaving myself with like 100 pictures, I go back and check how large the backup will be for my photos, and its only down to 4.5gb!!!! So confused as heck, as taking off 85% of that 5.1 GB should not only bring it down to 4.5 So, being anoyed with my phone,  I tossed it away for the time being. 

Later I start poking around again, and come across my usage tab. There I see something kind of strange. Spotify has 3.9 GB of data stored..... So this got me thinking. I know Apple's IOS is a very "locked down" OS, if you know what I mean. My understanding of it is IOS wont let apps go creating folders where ever you want in the file structure. So for an app like this, did it have to fool IOS into creating a folder in photos to dump the offline music? That, or where the heck do I have 4.5 gb more photos hiding on my phone? If this is what Spotify is doing, it kind of ruins the experience of iCloud as I can no longer use the free 5gb. I would remove spotify if I had the time to re download all of my offline playlists, but if no one knows a solution I may have to do just that. Thanks for the help. 

[Peter__]

Spotify will not be prentending to be photos, especially if it's data usage is being reported as a sensible figure itself.

Wondering did it actually remove the photos when you backed them up.