The paper deals with numerical modeling of electrothermal phenomena in 3D GaN core-shell light-emitting diode (LED) structures that were developed in the frame of GECCO project.1 The simulations investigate the influence of pillar dimensions on the LED work conditions. The inherent feature of such a design is the discrepancy between the internal contact footprint current density JFP and the current density on the junction active area JAA, which, at the same contact current, decreases when the pillar is taller. The simulations indicate that the decrease of JAA results in significant changes in the LED parameters. At the same diode current, i.e., constant light emission, it leads to the voltage decrease leading to the reduction of power delivered to the diode and, consequently, to the increase of its efficiency.