For very preterm infants, body size and composition (lean versus fat mass) may index brain growth and microstructural development. Among 85 very preterm infants at term equivalent age, we studied associations of body size/composition with brain magnetic resonance imaging (MRI) outcomes including total and regional brain volumes, and fractional anisotropy of white matter tracts. Larger body size and more lean--but not fat--mass were associated with larger brain volumes and higher fractional anisotropy of multiple white matter tracts. Lean mass accrual may index brain growth and development. MRI may be useful for studying effects of nutritional exposures on the preterm brain.