We provide here a mathematical model of size/context illusions, inspired by the functional architecture of the visual cortex. We first recall previous models of scale and orientation, in particular Sarti et al. in Biol Cybern 9:33–48, (2008), and simplify it, only considering the feature of scale. Then we recall the deformation model of illusion, introduced by Franceschiello et al. (J Math Imaging Vis 60:94–108, 2017b) to describe orientation related GOIs, and adapt it to size illusion. We finally apply the model to the Ebbinghaus and Delboeuf illusions, validating the results by comparing them with experimental data from Massaro and Anderson (J Exp Psychol 89:147, 1971) and Roberts et al. (Perception 34:847–856, 2005).