Using a rich sample created from the Longitudinal Survey of Australian Children, we investigate the extent to which the relationship between body size at birth and early childhood cognitive skills is mediated by physical development indicators. Consistent with existing evidence from other countries, we find a significant relationship between body size at birth and future development among Australian children as well, in terms of both weight and length. Accounting for progressive measures of physical developments and other confounding factors, however, indicates that only a small proportion of this association works through these pathways, while most of it remains persistent.