Final answer:
Strength training primarily causes muscle size to increase through hypertrophy, which is the enlargement of muscle cells due to additional structural proteins, rather than by increasing the number of muscle cells.
Step-by-step explanation:
The increase in muscle size observed from strength training is primarily the result of hypertrophy, which is an increase in the size of a muscle due to the addition of structural proteins to muscle fibers, thus increasing the cell diameter. Hypertrophy occurs when muscle cells are subjected to repetitive load-bearing or resistance exercise. This process involves the increase in the number of sarcomeres and myofibrils within the skeletal muscle cells but does not usually increase the number of muscle cells, which would be called hyperplasia. In the case of smooth muscle, such as that in the uterus, hyperplasia can occur alongside hypertrophy, but for skeletal muscles, like the ones usually involved in strength training, hypertrophy is the main mechanism for increased muscle size.