Two different types of muscle growth: sarcoplasmic vs. myofibrillar hypertrophy

A 'pumped up' muscle that isn't the strongest isn't a contradiction — it's physiology

Muscle growth in size can happen through two different mechanisms that produce different practical results. Myofibrillar hypertrophy is an increase in the number and size of the actual contractile protein structures inside a muscle fiber (actin and myosin), which directly generate force. More myofibrils means more potential strength for the same muscle volume.

Sarcoplasmic hypertrophy is an increase in the volume of sarcoplasm, the fluid part of the cell surrounding the myofibrils, which includes glycogen stores, water, and various enzymes. This kind of growth increases visual muscle size but contributes less directly to maximum strength compared to the same amount of myofibrillar growth.

Moderate-weight, high-volume training (typical of bodybuilding) is thought to stimulate the sarcoplasmic component more strongly, while very heavy low-rep training (typical of powerlifting) leans more myofibrillar — though both mechanisms are present to some degree in any type of strength training. This partly explains why athletes with similar muscle size can differ noticeably in absolute strength — their training emphasis engaged different growth components.