Changes in the relative 56Fe/57Fe isotope fractions due to Fe self-diffusion, that is active at the grain boundaries, can be monitored non-destructively by neutron reflectometry (NR) in the sub-nanometer length scale with atomic length scale precision. However, since grain boundary diffusion is inherently dependent upon the size of the grains and grain diameter variation perpendicular to the surface of multilayered specimens, it can often give ambiguous results in diffusivity. Here, we report on self-diffusion of Fe in bilayers, with two different thicknesses without concurrent grain growth during the annealing process. The thicknesses limit the size of the grains along the growth direction. Using NR, we find significant differences in the diffusivities as a function of annealing time in the bilayers. These results elucidate the microscopic mechanism of grain size dependent self-diffusion, which may redefine the regimes of diffusion attributed to grain boundary diffusion and volume (or lattice) diffusion.