Secondary iron-air batteries re-gained considerable scientific attention due to their excellent energy densities, pronounced environmental friendliness and exceptional reversibility compared to other metal-air batteries. In order to exploit the energy density of iron on full-cell level, the ratio between anode- and overall battery material should be as large as possible, aiming at practically competitive iron-air battery performances in the future. Therefore, here, we report the investigation of comparatively thick, pressed-plate, carbonyl iron-anodes and the distinctive attempt to further elucidate the processes behind the electrochemical formation. In order to do so, the electrode thickness-dependent charge-/discharge performance, the wetting behavior and the specific surface area of the electrodes were examined. In addition to the established dissolution and precipitation mechanism of iron, we propose that a gradually increasing number of electrochemically active carbonyl iron particles may be an additional source of active iron surface for the steeply increasing discharge capacity during the formation, which is particularly relevant for thick rather than thin electrodes. Furthermore, substantiated by cross-section SEM-images, we propose that the increasing number of active carbonyl iron particles is induced by microstructural changes of the electrode, hypothetically driven by hydrogen evolution during the formation period. Bound to the access of electrolyte, the process suggests the presence of active material on the outside and inactive, since non-wetted, material on the inside of porous carbonyl iron-anodes depending on their state of formation.