Nanocrystals - The strongest size

In polycrystalline materials with grain sizes in the micrometre range, strength increases with decreasing grain size. This is because dislocations pile up at the grain boundaries and, as the grains become smaller, the effect of dislocation blocking increases, thereby strengthening the material. But with grains in the nanometre range, the opposite behaviour is found. Why? Computer simulations show that the reverse effect arises primarily from sliding motions at grain boundaries.