From Skyshape Software:
MP3Resizer for Mac reduces the size of MP3 files by recompressing them with a lower bit rate. MP3Resizer for Mac makes it simple to optimize the capacity of your portable music player or media-capable phone. In a nutshell, it takes the MP3 audio data and recompresses it to reduce the total file size. It's perfect if you own lots of high quality media, and is especially effective for MP3 Audio books or Podcasts, which can be reduced drastically with little or no reduction in overall experience. Also you can make your MP3 files smaller so you can upload them faster.