My iPhone 4s (13.50GB capacity) has 0 bytes available which I know is odd?

My sister has the exact same phone and storage capacity as I do, but carries way more photos and apps, and still has so much space available (about 5.50GB). I've stripped my phone almost of every unnecessary app, photo, video, song, etc., and I still run out of space extremely fast. Can anyone tell me what's wrong or what I can do to fix it?

Anything that has space like that is never the exact. It is always 1 or 2 GB off. There's not anyway to fix that other than getting a iPhone with more space.