Barrel chest generally refers to a broad, deep chest found on a man. A man described as barrel chested will usually have a naturally large ribcage, very round torso, large lung capacity, and will potentially have lots of upper body strength. It can sometimes be a sign of acromegaly

Acromegaly

Acromegaly is a syndrome that results when the anterior pituitary gland produces excess growth hormone after epiphyseal plate closure at puberty...

 (a syndrome resulting from excess levels of Growth Hormone in the body).

Barrel chest also refers to an increase in the anterior posterior diameter of the chest wall resembling the shape of a barrel, most often associated with emphysema

Emphysema

Emphysema is a long-term, progressive disease of the lungs that primarily causes shortness of breath. In people with emphysema, the tissues necessary to support the physical shape and function of the lungs are destroyed. It is included in a group of diseases called chronic obstructive pulmonary...

. There are two main causes of the barrel chest phenomena in emphysema:

1. Increased compliance of the lungs leads to the accumulation of air pockets inside the thoracic cavity.
2. Increased compliance of the lungs increases the intrathoracic pressure. This increase in pressure allows the chest wall to naturally expand outward.