In the last decade, many techniques that use diffusion MRI to obtain an axon diameter estimate as micro-structural integrity index have been developed. However, recent studies showed that diffusion signal may be not sensitive enough to quantify axon diameters. In this study, we simulated a simplified model of white matter to evaluate the contribution of intra-axonal compartment to diffusion MRI signal in white matter. We found that, even in distributions with a small mean diameter, big axons still substantially contribute to the total axonal volume. We conclude that quantifications of human axon diameters from diffusion MRI may still be possible.