When I convert file to mp3, it gets bigger. Why ?

It may be because to play in your sony mp3 player will need bigger settings or quality of the sound, I explain: Mp3 files you can set the quality from 8 kbps to 320 kbps and the quality increases, with a sample rate of 2mgh to 5mgh, probably your sony mp3 player can play only from 128 kbps and you have your mp3 song on 64 kbps thats why it increases because it increases the quality of the song in order to play it in your sony mp3 player.