Background: Our goal was to analyze the relationship between patient size and the proportion of blood volume lost during spinal arthrodesis in patients with a diagnosis of adolescent idiopathic scoliosis, Scheuermann kyphosis, or cerebral palsy. We hypothesized that smaller patients (those with less blood volume) lose a greater proportion of circulating total blood volume during surgery. Methods: We reviewed a large, multicenter database, identifying patients with adolescent idiopathic scoliosis (1832), Scheuermann kyphosis (106), or cerebral palsy (196) who had undergone posterior spinal arthrodesis for spinal deformity. Blood volume (estimated from body weight) was used as a measure of patient size. Our primary outcome was the proportion of total circulating blood volume lost (intraoperative blood loss/blood volume, expressed as a percentage). Results: On multivariate analysis, there was a negative relationship between intraoperative blood loss/blood volume and blood volume in patients with adolescent idiopathic scoliosis (coefficient, -5.8; p < 0.001), Scheuermann kyphosis (coefficient, -2.5; p < 0.001), or cerebral palsy (coefficient, -20.3; p < 0.001), indicating that, despite adjustment for all other factors, smaller patients lost a greater proportion of their blood volume. In patients with adolescent idiopathic scoliosis or Scheuermann kyphosis, multivariate analysis showed that intraoperative blood loss/blood volume also increased significantly when the patient was male and with a greater number of levels fused. Conclusions: There is an inverse relationship between the proportion of blood volume lost during deformity correction surgery and size in patients with adolescent idiopathic scoliosis, cerebral palsy, or Scheuermann kyphosis. Level of Evidence: Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence.