Actually, "softness" DOES matter. In a sense. Physics 101: F=MA (force equals mass times acceleration) It's not just a matter of how massive the object, a pair of breasts in this case, is, but also how fast it accelerate (or decelerates). An object with "give" takes more time to stop, and thus applies less force. A brick will stop (almost) instantly with little distortion to the object. In this case, the massive mammaries will likely deform, causing them to decelerate more slowly, applying a much more pleasant force than bricks of equal mass.
Oh, I understood that part perfectly well. Their softness would indeed absorb some of the momentum upon impact. However, you'll note the comparison to a pillow. Even a relatively non-massive and soft object like a pillow can deliver significant force to the receiving object (i.e. the human head) if swung with sufficient force and carrying sufficient momentum.
Clearly, the breasts in this pic are massive indeed, to have overcome their own resiliency in delivering this epic titslap.
