MP3 to Wav converter info?

I have Incredimail, and we can attach music wavs to messages. I've gotten some emails with full songs in 'wav format, and the file sizes are only 1 to 3 MB per song. I found a mp3 to wav converter, and converted a song, but the file size was 42 MB, and when I lowered the compression, it was still 21 MB. Can someone tell me how to make wav files smaller, or what type of program I can use to convert full songs?