We live in an age when an increasing number of people are going to gyms and taking up sports or physical exercise, which is of course a good thing. Some of these people do this to improve their general health and fitness and hopefully live longer, others do it for enjoyment and recreation, while some do it to enhance the size and appearance of their bodies.
Of the people in the latter group, many will just use physical exercise, usually weight training and sometimes also cardiovascular exercise, to achieve this, and allow their bodies and muscles to expand naturally in response to the increased physical stress they subject them to.
Exercise is essential for muscle growth. When the muscles get placed under physical stress and pushed to their limits, they tear, and then they repair themselves and knit back together bigger and stronger than they were before.