The actinide contraction is the greater-than-expected decrease in atomic radii and ionic radii of the elements in the actinide series, from left to right.
[1] It is caused by the poor shielding effect of nuclear charge by the 5f electrons along with the expected periodic trend of increasing electronegativity and nuclear charge on moving from left to right.
About 40-50% of the actinide contraction has been attributed to relativistic effects.
This results in smaller than otherwise expected atomic radii and ionic radii for the subsequent d-block elements starting with 103, lawrencium.
[3][4][5][6] This effect causes the radii of transition metals of group 5 and 6 to become unusually similar, as the expected increase in radius going down a period is nearly cancelled out by the f-block insertion, and has many other far ranging consequences in post-actinide elements.