Aim: To examine the factors influencing height, weight, and body mass index (BMI) z-scores, and the relationship between them and motor performance, in boys with Duchenne muscular dystrophy (DMD). Method: This was a randomized, double-blind, parallel group trial involving 32 study sites across five countries. Height, weight, BMI z-scores, and clinical outcome assessments (COAs)—rise from supine velocity, 10-m walk/run velocity, NorthStar Ambulatory Assessment, and 6-minute walk test—were analysed in 4-year-old to 7-year-old boys with DMD randomized to 0.75 mg/kg/day prednisone, 0.75 mg/kg/day intermittent prednisone, or 0.90 mg/kg/day deflazacort in the FOR-DMD study. Trajectories were modelled using a linear mixed-effects model and correlations were explored through Spearman's partial correlations. Results: In 194 boys with DMD, higher height at glucocorticoid initiation was associated with slower growth (p < 0.001) and older age was associated with increased weight gain (p = 0.001). Glucocorticoid type and regimen influenced height and weight trajectories but not BMI. Changes in height and weight z-scores were negatively correlated with COAs (p < 0.05 in all cases). Correlations were weak 3 years after glucocorticoid initiation and moderate after 5 years (closer to the age of loss of ambulation). Interpretation: Changes in anthropometric measures after glucocorticoid initiation are associated with COA performance and larger correlations closer to the age of loss of ambulation. This emphasizes the need for weight management strategies and discussions that support treatment.