The field assisted sintering offers the unique possibility to consolidate materials while maintaining nanostructures. This method can be used to tailor grain sizes in sintered permanent bulk magnets. Since the physical properties of the bulk material changes, these changes have to be originated by variations of the electronic structure. In the case of iron rich magnetic materials, Moessbauer spectroscopy is an ideal tool to detect such variations. It is demonstrated that for nanostructured iron oxide bulk materials, the hyperfine interactions change due to the small size of grains below 80 nm.