Muscle size is defined by Hypertrophy, the enlargement of muscle fibers. Increased cross-sectional area of muscle fibers means a “larger” muscle and can increase the ability to develop force.
Now it gets tricky…stick with me fitness nerds. We have two main types of muscle fiber which you may have heard about, fast-twitch and slow-twitch. Hypertrophy however, does not occur uniformly between these 2 forms of muscle fibers. Our buddy the fast-twitch muscle fiber has shown to increase in size more than slow-twitch muscle fiber when tested with conventional resistance training in studies. Due to this fact it is argued that the amount of a person’s muscle mass is decided merely by the relative proportion of fast-twitch to slow-twitch. The more fast-twitch muscle you carry the more potentional for over-all muscle size and muscularity.