A light square against a dark background besides an equisized dark square against a light background shows a difference in apparent size such that the light square seems larger than the dark one. This illusion was studied for squares of 109 x 109 min arc. Their centres were located 90 min arc out of the centre of the fovea. The apparent size appeared to be dependent on the contrast between the square and the background and not on the mean luminance in the vicinity of the contour. The addition of a small white or dark line to the luminance step that constituted the border of the square changed the illusion dramatically. It is argued that this explains the observations where an inversion of the illusion for very small contrast values was found. The results are explained on the basis of a model with receptive fields of different sizes overlapping the same retinal location and having different sensitivities.