Final answer:
The circumference of the forearms is influenced by the circumference of each muscle fiber and the overall muscle bulk, which can be altered by processes like hypertrophy and atrophy, and not by the number of muscle fibers, as their quantity remains genetically constant.
Step-by-step explanation:
The difference in the circumference of the forearms is likely caused by the circumference of each muscle fiber combined with the overall bulk of the muscle rather than a difference in the number of muscle fibers. The number of muscle fibers in a given muscle is genetically determined and tends to remain constant. However, the size of these fibers can increase through a process known as hypertrophy, which results from factors like hormones, stress, and physical training which increase the production of myofibrils and sarcomeres within each fiber, thereby increasing mass and bulk of the muscle. At the same time, lack of use can lead to atrophy, which is the reduction in myofibrils and sarcomeres within the fibers, causing muscles to decrease in size.