While moving along the inner transition elements, the...

The decrease in ionic radii across the actinoid series (actinoid contraction) is more significant from element to element than the corresponding decrease in the lanthanoid series (lanthanoid contraction).

This occurs because the $5f$ orbitals are more diffuse and extend further from the nucleus compared to the $4f$ orbitals. As a result, the $5f$ electrons shield the increasing nuclear charge less effectively than the $4f$ electrons.

Due to this poorer shielding by $5f$ electrons, the effective nuclear charge experienced by the outermost electrons increases more rapidly across the actinoid series, leading to a greater contraction in size.

Answer: The $5f$ electrons shield the increasing nuclear charge less effectively than the $4f$ electrons.