You need to change the bitrate of the MP3... if you change it from 192 kb/s to 96 kb/s it will use half the space but won't sound as good. Anything less than 96 will sound a bit ropey (for music anyway).
You can use this program to change bitrate (use the MP3->MP3 option)...
http://cdexos.sourceforge.net/download.html but there are others if your Google search for them.