To fully understand your memory usage you need to look at your phone usage. Some functions that chew up loads of memory are obvious i.e. Taking photos, videos, saving music and documents to the phone (I would strongly advise opening a onedrive account if you don't already have one.....free storage for all those pics, videos and docs). Other functions that can contribute massively to storage issues include web surfing and playing games. Temporary internet files can take up hundreds of megs. Equally, games that create restore points or save where you are when you exit can cause havoc with your memory. Examples; minion rush is 43megs, but is taking up 360megs on my phone. Music and video apps are similar.......youtube and Spotify are only 10-20megs in size but take up over a gig each on my phone. Run Storage Check in settings, it'll help you to figure out what's eating your memory.