You're solving for force when the thing that causes the damage is (very rapid) acceleration. The total acceleration is the initial difference in speed between the two things that collide and the proportion of the acceleration applied to each object is inversely proportional to their relative masses.

If they have the same mass then they each take half. If one of them is 19 times heavier then the other one takes 95% of the acceleration. If one of them is six hundred trillion times heavier then the other one takes ~100% of the acceleration, but this is still only <5% more than taking 95%.