Reason of lanthanoid contraction is:

Lanthanoids are the $f -$block elements and $4f$ series element. The orbital size of the $4f$ is much bigger due to which electrons are spread in the large area, these electrons not able to shield charge from the nucleus. As a result, the poor screening effect or poor shielding effect of the $4f$ sub-shell. There is regular decrement of the atomic radius on increasing the atomic number.