We demonstrate that the concept of mesoscopic misfit, rather than macroscop
ic misfit, should be used to describe the atomistic processes in the early
stages of metal heteroepitaxy. Atomic scale calculations reveal the drastic
effect of the mesoscopic misfit on strain in Co islands on the Cu(001) sur
face. We show that atomic motion on strained islands is strongly affected b
y the mesoscopic misfit and depends on the size of the islands. The diffusi
on coefficient on top of small islands is found to be larger by 2 orders of
magnitude than that on large islands.