Lanthanide contraction :

  There is a decrement in the size of the atoms and ions of the $\mathrm{f}- \mathrm{block}$elements with increasing atomic number from  $\mathrm{La}$  having atomic number $57$ through $\mathrm{Lu}$  having atomic number $71.$ Weak screening effect of the $4\mathrm{f}$- electrons called Lanthanide contraction