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.
http://omion.dyndns.org/mp3packer/mp3packer.html
so i call it a lossy optimizer as its NOT bit identical but if both old and new file converted into WAV format it will be identical 100% it reduce up to 5% depending on the original MP3 and its producer (fhg, lame, gogo,..)
this if someone interested in mp3-reducer like what dont with JPG..