I find that a bag thats very heavy makes you concentrate on hitting most. I find when it oes swing it's temporary, where as a medium weight bag will provide a certain degree of movement, as an opponent would.

A medium weight bag allows me to envision "steering the opponent."

I want an opponent to move to my right I throw a left hook, he moves right I step to my left, he's entirely to my right.....Things like that you know.