Photostrictive materials have a growing interest because of their great potential as light-driven actuators, among other optomechanical applications. In this context, the optical control of macroscopic strain in ferroelectrics has recently attracted remarkable attention as an effective alternative to the conventional electric control of strain. Here, a clear correlation between optical absorption and light-induced strain in polycrystalline BaTiO3 is shown. Specifically, the grain size and the sample thickness dependence of optical absorption when the material is irradiated with energy photons lower than the band gap evidence that light absorption at charged domain walls is the core of the observed photo-response in ferroelectrics. The photoinduced electronic reconstruction phenomenon is proposed as the primary physical mechanism for light absorption at charged domain walls. Results open a new pathway to designing ferroelectric-based devices with new functionalities like thickness gradient-based photo-controlled nanoactuators.