A quite enjoyable picture that roughly depicts a part of the musical devices evolution. This topic emerged from a discussion with Laurence, one of the design student I supervise for a masters thesis. As opposed to the tape and CD cases - where the device's shape was constrained by the shape of the tapes and CD - the MP3 player required some thinking about its overall shape.
The content is reduced to something smaller than the CD and the tape (I wouldn't go as far as stating that it's immaterial because the MP3s files are located on some sort of electronic component IN the device). However, it raise the issue of the general shape regardless of the content.