The 5f orbitals (in actinoids) have a poorer shielding effect than 4f orbitals (in lanthanoids). Thus, the effective nuclear charge experienced by electrons in outer shells in case of actinoids is much more that experienced by lanthanoids. As the effective nuclear charge experienced is high, the electrons are attracted with much force, hence the size of the atom decreases. Hence, actinoid contraction is greater from element to element than lanthanoid contraction.