The performance of microbial fuel cells and other related microbial electrochemical processes is seen to deteriorate severely when they are scaled up. This crucial problem is addressed here by comparing the kinetics of microbial anodes with projected surface areas of 9 and 50 cm(2) under well-controlled electrochemical conditions. The microbial anode kinetics were characterized by low scan rate voltammetry. The 9-cm(2) anodes showed Nernstian behaviour, while the 50-cm(2) anodes showed significantly lower performance. The distribution of the electrostatic potential in the experimental set-up was modelled numerically. The model predicted the general trend of the voltammetry curves recorded with the 50-cm(2) anodes well, showing that part of the performance deterioration was due to ohmic drop and to non-uniformity of the local potential over the anode surface. Furthermore, the biofilm presented slightly different electrochemical characteristics when grown on the 9-cm(2) or 50-cm(2) anodes, and the difference in local potential over the 50-cm(2) anodes induced spatial heterogeneity in biofilm development. The effect of local potential on biofilm characteristics was an additional cause of the lower performance obtained with the 50cm(2) anodes. In the current state of the art, the soundest way to design large-sized microbial anodes is to adopt the dual main aim of minimizing the ohmic drop while keeping the most uniform possible potential over the electrode surface. Modelling potential distribution inside the reactor should make an essential contribution to this. (C) 2017 Elsevier B.V. All rights reserved.