Not possible ... use AAC (MP4) encoding if you want to make your sound files smaller than MP3's .... the problem is that MP3's are universal and MP4 are only used on the iPod.
well you can make them smaller by reencoding using a lower bitrate, but you'll lose quality. A better bet is to reencode using a variable bitrate - you'll get a smaller file with about the same quality as the original (if it was encoded with a fixed bitrate that is). You can even try saving in different formats like ogg which generally produce smaller files than mp3s at similar quality