Background Women with large breasts frequently experience upper torso pain secondary to their breast size. Evidence is lacking on the underlying causes of this pain. This study investigated whether upper torso pain and musculoskeletal structure and function differed between women with large breasts and women with small breasts. Methods A linear regression, adjusting for body mass, compared the upper torso pain, thoracic flexion torque due to breast mass, thoracic kyphosis, shoulder active range-of-motion, and scapular retraction muscle strength of 27 women with large breasts (bilateral breast volume > 1200 ml, age 45.9 y SD 9.9 y, BMI 29.0 kg/m 2 SD 3.8 kg/m 2 ) and 26 women with small breasts (bilateral breast volume < 800 ml, age 43.8 y SD10.9 y, BMI 23.3 kg/m 2 SD 2.9 kg/m 2 ). Findings Women with large breasts reported had a higher upper torso pain score (46.6, 95%CI 33.3-58.0 versus 24.1, 95%CI 12.5-37.8), accompanied by a larger flexion torque (5.9 Nm, 95%CI 4.5-5.8 Nm versus 0.9 Nm, 95%CI 0.8-2.4 Nm), greater thoracic kyphosis (34° 95%CI 31-38° versus 27° 95% CI 24-31°), decreased shoulder elevation range-of-motion (160° 95%CI 158-163° versus 169° 95%CI 166-172°), and decreased scapular retraction endurance-strength (511.4 s, 95%CI 362.2-691.3 s versus 875.8 s, 95%CI 691.5-1028.4 s) compared to the women with small breasts. Interpretation Differences in the upper torso posture, range-of-motion, and muscle strength of women with large breasts provides insight into underlying causes of their musculoskeletal pain. This information can be used to develop evidence-based assessment and treatment strategies to relieve and prevent symptom progression.