We study comparatively some characteristics of photodarkening and photoexpansion in chalcogenide glasses. In thicker samples, both grow more slowly, but the thickness variations are different. The feature can be understood with an idea that the two phenomena originate from a common atomic displacement, while the expansion in thick films suffers marked delays, due to viscous transfer of photogenerated internal stress to sample surfaces. Densities of the common atomic site are estimated at 0.5–1 at.%, the situation corresponding to single isolated (uncorrelated) centres in medium-range structures, which provides the quasi-stability of illuminated states. We also discuss relations between the darkening, the expansion (density reduction), and a refractive-index increase.