mp3 files are already compressed, so to reduce the filesize further you're going to have to reduce some of the quality settings for the mp3. Find out what bitrate the file is (like 320kbps, 192kbps...) and then try converting to an mp3 with a reduced rate (like 128kbps, 96kbps...). Use the joint-stereo option also, of if appropriate select mono which will further reduce the filesize.