I have a 516 mb micro mini sd. It won't hold more then 50 songs. What am I doing wrong ? should it hold more ?

I got the micro mini so I could play songs on my cell phone ( A motorola E815). I thought the 516 MB would hold around 500 songs but after around 50 songs the card is full. Am I doing something wrong ? I am using mp3's because it wouldn't play WMA's.