Discover how to build a massive and impressive chest (pectoral muscles). Your pectoral muscles are one of the largest muscle groups in your body. Also, to develop your pectoral muscles, you must use compound and isolated lifts to build, tone, and shape them. The pectoral muscles are complex and have many different and unique parts. The upper, lower, inner, and outer parts of the pectoral muscles, plus the pectoralis major and pectoralis minor muscles, make up the muscle.