The swimmer has a barrel-chested build which helps him stay balanced in the water.

barrel-chested 是由名词 barrel（桶）和形容词 chested（有...胸部的）组成的复合形容词。barrel 指"桶"，形容形状圆而宽大；chested 来自名词 chest（胸部）加上 -ed 后缀，表示"有...胸部的"。 barrel-chested 形容人的胸部宽大圆润，像桶的形状一样，这种体型在游泳时有助于保持平衡。