1) F=ma rules impact force imparted and so having a heavier clubhead can increase the force you put on the ball if you are able to accelerate it at least close to what you would a lighter clubhead (this speaks to your feeling of mashing it more)
2) one of the things that makes the longer irons harder to hit is the clubhead is lighter and it is harder to "feel" where it is (this speaks to your tempo and lag help)