The moment of inertia of an object about a given axis describes how difficult it is to change its angular motion about that axis. Therefore, it encompasses not just how much mass the object has overall, but how far each bit of mass is from the axis. The farther out the object's mass is, the more rotational inertia the object has, and the more force is required to change its rotation rate.

Which means in the case of the EDM - Putter:

The more weight you have on the outer most of your Putter head the less pissible it is to rotate the Putter during the impact.