There are two types of muscle growth that affect the look of a muscle.
1. Sarcoplasmic Hypertrophy: This is growth caused by the increase in fluid (sarcoplasm) within a muscle cell. This is a fast way to increase the size of a muscle, but since sarcoplasm is a fluid and can't contract...it won't make the muscle significantly stronger.

2. Myofibrillar Hypertrophy: This is actual muscle fiber growth. This adds a lot less to the size of a muscle than sarcoplasmic growth, but since muscle fiber can contract...it will make the muscles significantly stronger.