Q. Aren't both weight and strength governed by the cube function, not one cube and the other square as you state in your book? Weight is proportional to volume. Muscle mass is proportional to volume. Isn't muscle strength proportional to muscle mass?
A. Muscle strength is proportional to the cross-sectional area of muscle. It makes no difference how long the muscle is. A muscle’s strength depends on the area in tension and not its volume.
You could try a little experiment. Load something until it breaks. A piece of string could represent some muscle perhaps. Then lengthen the string and try again. I think it will break at a similar load. The length makes no difference to its strength.