Mammographic % density, the proportion of fibroglandular tissue in the breast, is a strong risk factor for breast cancer, but its determinants in young women are unknown. We examined associations between magnetic resonance imaging (MRI) breast-tissue composition at age 21 years and prospectively-collected measures of body size and composition from birth to early adulthood, and markers of puberty (all standardized), in a sample of 500 nulliparous women from a pre-birth cohort of children born in Avon, England, in 1991-1992 and followed up to 2011-2014. Linear models were fitted to estimate relative change in MRI % water, which is equivalent to mammographic % density, associated with one standard deviation increase in the exposure of interest. In mutually-adjusted analyses, MRI % water was positively associated with birth weight (relative change = 1.03 (95% confidence interval: 1.00, 1.06)) and pubertal height growth (1.07 (1.02, 1.13)), but inversely associated with pubertal weight growth (0.86 (0.84, 0.89)) and changes in dual-energy x-ray absorptiometry % body fat mass (e.g., 0.96 (0.93, 0.99)), for change between ages 11-13.5 years). Ages at thelarche and menarche were positively associated with MRI % water, but these associations did not persist upon adjustment for height and weight growth. These findings support the hypothesis that growth trajectories influence breast-tissue composition in young women, whereas puberty plays no independent role.