Internal melting and differentiation of Ganymede and Callisto may have caused an increase in the surface area of these bodies early in their histories of up to 5--7%. Subsequent refreezing of internal liquid water due to solid state convection in an ice crust should not have caused significant surface area changes. Expansion due to differentiation may have caused formation of grooved terrain on Ganymede. These calculations suggest that grooved terrain formation is essentially a replacement and/or deformation process, with no more than about 15% of grooved terrain actually being new material. The absence of grooved terrain on Callisto may be due to the effects of a thicker crust and a lower expansion rate.