Exactly what Skalla said.
Think of it as me throwing a baseball at a wall. Normally it would bounce off, if it were a brick wall. However, if I increase the momentum and force behind it enough it could break through the wall, despite teh wall holding more weight and density than the ball itself. Of course that would never happen realistically. But seeing as this is fantasy we're doing, I could implement enough force and momentum to shatter through the wall with ease.
Like I said, I would recommend researching momentum a bit since I fight using kinetic energy and momentum forces.