Some polymers exhibit very high electromechanical activity, and there is a lack of physical understanding of the mechanisms at the origin of this behavior. In amorphous or slightly crystalline polymers, piezoelectric effect is negligible and the contributions to electrostriction are quadratic function of the applied electric field. These contributions are extrinsic and intrinsic, namely, (i) the electrostatic pressure resulting from the two electrodes attraction (Maxwell effect) and (ii) dipoles-electric field interactions resulting in a mechanism so-called electrostriction. The later contribution can reach much higher value, i.e., by a factor 1000, than the Maxwell effect in some polyurethanes. On the other hand, dipoles-dipoles interactions are known to play a negligible role in homogeneous media. In this work, it is shown that both heterogeneities of local stiffness and dielectric constants are responsible for this unexpected behavior. Nano-heterogeneities may result from phase separation in block copolymers, or can be introduced by dispersing nanoparticles within a matrix. A theoretical approach based on dipole-electric field gradient is developed and numerical data are compared with experimental data from the literature.