Why does the size of the ventral horns of the spinal cord vary along its length?

(1) through the spinal nerves that attach to it, the spinal cord is involved in the sensory and motor innervation of the entire body inferior to the head; (2) through the ascending and descending tracts traveling within its white matter, the spinal cord provides a two-way conduction pathway for signals between the body and brain; (3) through sensory and motor integration in its gray matter, the spinal cord is a major center for reflexes