1. As we move from one element to another in 4f series (Ce to Lu) the nuclear charge increases by one unit and an additional electron is added into the same inner 4f sub-shell.
2. 4f sub-shell have a diffused shapes and therefore the shielding effect of 4f electrons are relatively poor. Hence, with increase of nuclear charge, the valence shell is pulled slightly towards nucleus.
3. As a result, the effective nuclear charge experienced by the 4f elelctoms increases and the size of Ln3+ ions decreases.