In low glass-transition temperature photorefractive materials, push-pull chromophores induce photorefractivity essentially via two mechanisms: optical birefringence and Pockels electrooptic effect. A 2-state 2-form description of these molecules indicates how the molecular structure can be optimized to maximize the photorefractive effect. At the optimum structure the two contributions have opposite signs. For the molecular sizes used currently the optical birefringence should dominate photorefractivity. Lengthening the conjugation path leads to a steeper increase of the Pockels electrooptic effect. Yet for the molecular sizes examined in this work, this contribution remains smaller than the optical birefringence at the optimum structure. Huge photorefractive responses could by attained if the molecular structure can be tuned towards the cyanine limit while increasing the molecular size.