Why does my Iphone 4s have 6 gigs taken up by photos?

I only have 860 photos on the entire phone and a few very short videos. I have checked the files in the DCIM folder to see if there were some in there that were not showing but there's only the 860 listed. I have no idea how 860 photos could use up that much memory. My friends Iphone has over 2000 pictures and only 2.5 gigs are taken up.

Perhaps your pictures are at a higher resolution.