I had this problem! Like 80% of my phone was "other." I couldn't take videos, only store like 20 pictures at a time. It was pissing me off.
I googled, and something is probably fucked up/corrupted. I reset my phone and I have SO much more space now. Like right now I have something like 200 pictures and a ton of videos and its still not full.
So try to reboot it. I'll try to find the link to what I did.
If I reset it to factory settings, will I lose everything?
I just deleted whole conversations. The one with my best friend and DH freed up about 2gb. But that was every photo and message we had sent over a very long period of time.