Explain briefly the cause of Lanthanoid contraction

Hi student
The major cause for lanthanide contraction is due to the inappropriate shielding of the 4f electrons due to the improper shape of the f-orbitals. As the atomic number increases in the lanthanide series, for every proton in the nucleus the extra electron goes to fill the 4f-orbitals.
This results in gradual decrease in the size of lanthanides with increasing atomic number. The decrease in size is not regular throughout the lanthanides. A rapid decrease is seen only in the first six elements compared to the rest of the elements.