My iphone 4s storage? (16gb)?

Whenever i synced my iphone i would have about 11-10 gb of space left (100 songs, 20 apps, 600 photos) but when i recently synced my phone i only had about 5 gb left? It seemed like i was losing the most storage from my photos which i don't understand because i only had 600 and itunes said it took up more than 6 gb of storage so i deleted about 350 of my photos and my photos still take up 5 GB by itself when i only have 250 pictures

The software takes up a lot of space even when the phone has nothing on it.