All of this depends on the actual parameters we are talking about > last radius goes at the cube root of energy, blast area goes at the square of blast radius.

All of this depends on the actual parameters we are talking about > last radius goes at the cube root of energy, blast area goes at the square of blast radius. Only up to the point at which the blast wave touches atmosphere then strange things happen because the blast is now being tunneled into a vacuum. This likely reduces the blast of very big asteroids by channeling most of the energy back into space. However blast area isn't the only concern. An ice age can be triggered if enough matter is blasted into the upper atmosphere. You might increase the blast area by breaking the asteroid into smaller pieces but completely eliminate the risk of depositing light blocking particles in the upper atmosphere. It is by no means guaranteed that if you break a big asteroid into eight pieces, they will all hit. The energy to split an asteroid may confer sufficient momentum that small if not all the pieces miss the Earth.