This study has focused on how assimilation and contrast correlate with the generating processes of various optical illusions. First of all, we defined the meanings of assimilation and contrast from two viewpoints, namely, phenomena and mechanisms. We then examined the characteristic appearances of the Delboeuf illusion as a typical size illusion caused by assimilation and contrast, by referring to major studies on this illusion in Japan. To confirm the different size-judgment processes operating in size illusions of concentric circles, quadrilaterals, lines, and angles, we explored the magnitude of illusion, including peak and conversion from overestimation to underestimation. The consistent occurrences of peak and conversion indicated the antagonistic correlations of assimilation and contrast in the generation of these illusions. Manipulation of figural unification using solid and non-solid segments had no effect on these illusions, which were constructed almost by their own specific contour-patterns. Finally, we tried to incorporate assimilation and contrast into our proposition that the combination of three factors (angle/direction, space/position, and assimilation/contrast) explains the generation of many optical illusions.