MP3packer is a program which can rearrange the data within an MP3 to fulfill specific goals. By default, the program generates the smallest MP3 possible (with the least padding). However, many people also use it to turn VBR files into CBR for use with players which don’t support VBR.
It can save space by storing frame data in the smallest possible frame. Usually MP3s are already stored in the most efficient way possible. However, for high-bitrate CBR files (like -b 320) there can be a lot of wasted space. ~ HydrogenAudio