Originally Posted by Slacker
Its purely physics. Strength has nothing to do with it.
Force = Mass x Acceleration
Example: A box weighs 2lbs. If you throw it at 30mph its going to generate the same amount of force every single time you throw it. The only way it gains more force is if the weight of the box is increased or the speed increases.
But you're forgetting about the delivery of the force. If the box is fairly soft, and partially crumples on impact, while it may still deliver the same amount of force in total it will do so at a slightly slower rate than if the box were completely solid with no "give" in its structure.
Now, it's an incredible over-simplification but it may be necessary to illustrate the point - two guys, same mass, throw a punch but one has a weak arm and one a strong arm. The boxer with the strong arm hits with a very solid, rigid arm that successfully transfers all of his power to his opponent, in an instant. The other guy hits, but instead of delivering all of the force at the end of a solid arm, his weaker arm "buckles", and force is absorbed by his elbow joint, shoulder joint etc.
Of course in reality a great deal more parts of your body than just your arm are important for delivering the force of your punch. There are many links along the chain that could be weak and reduce the impact.