The efficiency of interlayer mass transport determines the growth mode and film quality in molecular beam epitaxy. In this paper we report potential energy barriers (FEB) to interlayer diffusion for Pt and Ag homoepitaxial growth on fcc(111) surfaces, as calculated using the corrected effective medium theory. Various island structures were considered. The island sizes ranged from 3- to about 60-atom islands and to various steps (@'infinite@' large islands). We found that jumping directly over the island edge has a much higher FEB than does the so-called displacement-exchange mechanism. Exchange at edges with kink sites also had a higher or comparable FEB to those at the straight (perfect) edges, contrary to previous speculations [M. Henzler, T. Schmidt and E.Z. Luo, in: The Structure of Surfaces IV (World Scientific, Signapore, 1994)]. The FEB depended strongly on the local atomic arrangement but was insensitive to the global island size and shape as long as the island edges were at least five atoms long. For the displacement-exchange process, the FEB did not decrease monotonically with decreasing island size over the entire island size range. For very small islands of less than ten atoms the FEB increased abruptly by an order of magnitude. This qualitative behavior was exhibited by both Pt and Ag systems but the two differed quantitatively for island sizes above ten atoms. We discuss the relevance of these results to the experimental observations, i.e., the reentrant growth in Pt, layer-by-layer growth in Ag induced by surfactant or by high-density of islands 2, as well as the different growth behaviors found in the two systems.