We examined growth trajectories of hippocampal volume (HV) in early childhood in a longitudinal cohort of male and female participants with autism spectrum disorder (ASD) and typical development (TD), and investigated HV in those with large brains. Relations between factors potentially associated with hippocampal size and growth were investigated.Participants received 1-3 structural magnetic resonance imaging scans between ages 25-80 months (Unique participants: ASD: n =200; TD: n =110; total longitudinal scans, N = 593). HV growth during this period was examined using mixed effects linear models. Associations between early HV and growth rates, and IQ and adaptive functioning were evaluated.After accounting for cerebral hemisphere volume, male participants exhibited larger left and right HV than female participants. Hippocampal growth rates did not differ by sex. In children with larger hemisphere volumes, male and female participants with ASD had relatively larger HV than TD of similar hemisphere volume. This effect was present in a broader group than just those having disproportionate megalencephaly (male participants with large cerebral volumes relative to body size). Right were larger than left hippocampi in both groups and sexes. Right versus left volume differences were greater for ASD. After adjusting for hemisphere volume, male participants with ASD showed a significant positive association between right hippocampal growth and adaptive behavior.HV was relatively greater in ASD in analyses adjusting for hemisphere volume, while only subtle differences were observed in HV and growth between ASD and TD in unadjusted analyses, suggesting ASD involves atypical coupling between HV and brain size.