Why do photos take up so much space?

So i have a 32gb iPhone 4S and one day my memory was full. I plugged it into itunes and saw that i had 17gb worst of photos. 75% were just worthless screen shots from facenook pictures so I decided to import my pictures, delete them all from my phone and go through all of it then pick out ones that were important e.g. Holiday and family. There was about 900-1000 pictures and some videos and when i put it in, it took up 14gb? I did a test and put in 400 different photos and that only took up 0.15gb. Why did this happen and how can i fix it, thank you.

It depends on the settings you have the pictures under, its taking 8mp pics and that's a lot of quality and pixels to save onto your phone