In the worst case pawns can be quite demanding - because of underpromotion - there can be up to 12 moves available to a pawn - more than a knight or a king. Of course it is possible to design a scheme where moves take a variable number of bits - I discuss some promising methods in the article. But you will always need 8 bits for some moves (information theory says so) and the beauty of my "sweet spot" scheme is that it simple and quick, whilst still offering reasonable compression.