unity ar scale

                
                        ARSessionOrigin arSessionOrigin = FindObjectOfType<ARSessionOrigin>();
float contentScale = /* inverted (?) scale factor for the object */;
Transform objectToScale = /* parent transform for what you are scaling */;
arSessionOrigin.MakeContentAppearAt(objectToScale, objectToScale.position);
arRootObject.transform.localScale = new Vector3(contentScale, contentScale, contentScale);