The main reason of lanthanide contraction is, poor shielding of 4f electrons on the outer most shell electrons. As f orbital is defused nature so shielding of 4f electron is imperfect that is why nucleus can attract outer most shell electron easily and the size of atom decreases. In lanthanides, there is poorer shielding of 5d electrons by 4f electrons resulting in greater attraction of the nucleus over 5d electrons and reduction of the atomic radii.