Why does my music take up so much room on my iPhone?

I've been recently frustrated with how little space my iPhone had left considering how much content I have on my phone (2200 songs at 17.9GB, 1200 pictures at 4.6GB, 29 apps at 3.02GB, etc.). I compared it to my sister's identical iPhone, which has much of the same content. However, her phone has 2100 songs at 8.8GB, 3700 at 3.4GB and 46 apps at 3.4GB. Much of her music was downloaded directly from my MacBook. Why is my phone so full while hers still hasn't reached half-capacity?

Her music was probably convered to 128kbps AAC audio, which is a lot smaller than the regular file. The only downside is that the music quality sounds awful.