I only have a few photos on my iphone but it takes up a lot of storage?

I had about 700 photos until my iPhone told me that I had no more storage, and my photos took up 9.7 GB. I deleted some photos until I only had 537 photos, but my photos still took up 9.7 GB! Is there any reason why I didn't get more storage? (I have ios8)

I have 13 photos and 3 slo no videos and it says my photo library takes up 5.6 gigabytes. How can I lower it?