We improved the atomic size factor term that can be used in extended Miedema's models for calculating formation enthalpies of binary transition-metal compounds. With this new prefactor, the effects of the atomic size difference between two dissimilar transition metals can be taken into account in a more insightful way. Calculations using the present model were compared with experiments as well as with the original Miedema's model, showing that it can effectively improve the prediction precision of formation enthalpies. (C) 2011 Elsevier B. V. All rights reserved.