Atomic-scale computer simulations have previously identified a deformation mechanism, which becomes important in nanocrystalline metals with grain sizes below 10—15 nm. Instead of proceeding through dislocation activity in the grains, the deformation occurs by slip events in the grain boundaries, leading to a reverse Hall-Petch effect, i.e. a decrease in hardness with decreasing grain size. In this paper, the consequences of this shift in deformation mode are investigated for systems subjected to large strains in a cyclic deformation pattern. In most coarse-grained metals, severe plastic deformation leads to grain refinement. Indeed, severe plastic deformation is often used to generate nanocrystalline metals with grain sizes down to hundred nanometres. The simulations indicate that these processes are suppressed in sufficiently small grains, and instead the sliding events in the grain boundaries dramatically enhance diffusion processes, and lead to grain coarsening as the deformation proceeds.