Lanthanide contraction is the gradual decrease in the atomic and ionic size of lanthanoids with an increase in atomic number.
Causes of lanthanide contraction:
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 imperfectly 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. This causes lanthanide contraction.