The lathanoid contraction relates to

Lanthanide contraction is the gradual decrease in the atomic and ionic size of lanthanides with an increase in atomic number in d and f block elements. The cause for lanthanide contraction is explained as follows, with an increase in the atomic number, the positive charge on nucleus increases by one unit and one more electron enters same 4f subshell. The electrons in 4f subshell poorly shield each other. Shielding in a 4f subshell is lesser than in d subshell. With the increase in nuclear charge, the valence shell is pulled slightly towards the nucleus. Due to this the atomic or the ionic size decreases instead of increasing.