Proton-conducting, acceptor-doped, BaZrO3 based perovskite oxides are one of the most promising family of compounds for applications as, eg electrolyte materials, in the intermediate temperature ranges. While the proton conducting properties are usually studied through the influence of the chemical composition and hydration rate of the materials, we here focus our investigation on the influence of the microstructure, with a focus on surface diffusion effects. By evaluating the nature of the diffusion processes in nano- to microcrystalline BaZr0.9In0.1O3H0.1 samples, and in particular the extent and role of the surface diffusion processes, we aim to address the rising interest in nanocrystalline proton conductors and in sintering additives that often segregate as nanoparticles on the surface of the materials. The results should be generalizable to other materials of the perovskite family.