Transverse and volume magnification effects are described for the case of a single lens used to relay the object space in off-axis holography of micro-objects. Using a single calibration of the transverse magnification at a mean longitudinal image distance, we derive relationships for transverse and volume magnifications for a given control volume. The validity of simple limiting solutions in relevant physical conditions is also discussed. Using the knowledge of the transverse magnification at two longitudinal locations, we determine a simple solution for the magnification calculation at any third location. The exact solutions for the transverse and volume magnifications are then obtained in a simpler manner.