Like any technology, it gets smaller, cooler, quieter and cheaper to manufacture as time goes by. The current PS3 is about as small as they could get it while being cost effective, eventually the cell, gpu and blu-ray diode technologies will have advanced enough so that they are produced as much smaller devices.
Unless you are built by Microsoft... Then make it the same size give it a nasty paint job and make it a little louder for good measure !