There are more exact things, but at a bare minimum "damage" needs to fill the volume it's interacting with, and the volume of a sphere goes up by the cube of di

There are more exact things, but at a bare minimum "damage" needs to fill the volume it's interacting with, and the volume of a sphere goes up by the cube of distance. It's what led to the development of the smart bomb - they figured out that it's a losing game to increase the power of a bomb - that cube root is painful. So instead they made small, but more accurate, bombs. It's also why the nuclear bomb is not as useful as people think: It's strong, but a large number of accurate, small, smart bombs works better to destroy your target (as opposed to digging holes in the ground).