Force is equal to mass x acceleration. The heavyweight is packing both greater acceleration and mass than the tissue paper. When his fist comes in contact with the paper, the tissue reacts by exerting its own opposite force, providing a tiny bit of resistance until the fist breaks it apart and passes through. The fist, although by a a very tiny amount, is decelerated (slowed down) by the paper before it breaks through.
The gravitational force is inversely proportional to the square of the distance between their centers. So the force is greatest when the distance is zero.
