Also note that MP3 is a lossy format. During the conversion, sounds get thrown out, so when you go back to WAV format, you may notice a smaller file size. That is normal. This is also why you don't convert from a lossy format to anything else. For instance, take an MP3 and convert it to WMA and listen to it. It will sound like junk compared the the original sound/song.
-Sephiroth