Answer:
Jim Fitch's constant exercise caused hypertrophy in the muscles of his arms and chest, making those muscles bigger.
Step-by-step explanation:
As stated in the question above, Jim Fitch was not satisfied with his physique and therefore decided to "pump iron", which can be considered a muscle endurance exercise. This is because this type of exercise allows Fitch to do several repetitions that aim to stimulate resistance, causing an increase in the strength of the muscles of your arm and chest. This will cause a hypertrophy in the muscle cells of that region which will result in a significant increase in the size of these muscles, leaving them well defined.