iphone 6 storage problem

I have a 16gb iphone 6 and have 977 photos but it says that my photos are taking up 6.5gbs of storage and only 83.mbs of space left on my phone. I only have 16 apps and 36 songs and none of them take up too much space. I hooked by phone up to itunes to backup my phone and itunes says that my phone has 3705 photos and the photos are only taking up 2.46gbs of storage and that i have 2.03 gbs of space left. Why does the itunes storage differ so much from the one on my phone and how to i keep space on my phone with out having to continue to delete photos off of it?