The switching time and the coercive field in ferroelectric BaTiO3are known to depend on crystal thickness. A model along the following lines is proposed to explain the experimental results: on the crystal surface there is a thin layer of electrically biased and mechanically strained BaTiO3which has a much smaller dielectric constant than the bulk. It is proposed that the electrical bias and the mechanical strains in the surface are caused by a Schottky exhaustion layer. This model and the model proposed by Drougard and Landauer are discussed and their predictions are compared with experimental results.