How to free up more GB on iPhone 4s?

Recently I noticed my iPhone 4s is running low on memory. I did take more photos and videos. But I noticed something strange. My sister has a 16GB iPhone 4 and I have a 16GB iPhone 4s. She has 568 songs (3.5GB) and I have 366 songs (4.1GB). No songs of mine and hers are over 256 in bit rate. She has 366 photos (0.94GB) and I have 205 photos (4.2GB). Why is this?

Her pictures have lower resolution than yours.

The higher the resolution, the bigger the file size.