Causes of lanthanide contraction:-
1. as the atomic number of lanthanoids or 4 f block elements increases the positive nuclear charge increases and the correspondingly electrons are added to the prepenultimate 4f subshell
2. The attraction of nucleus on 4f electrons increases with the increase in atomic number.
3. The outer 11 electrons namely, 5s^2, 5p^6, 5d^1 and 6s^2 do not shield inner 4f electrons from the nucleus
4. There is imperfect shielding of each for 4 f electron from other 4f electron
5. as compared to d subshell the extent of shielding for 4f electrons is less
6. due to this cummilative effects 4f electrons experience greater nuclear attraction and hence valence shell is pulled towards the nucleus to the greater extent decreasing atomic and ionic radii appreciably
7. From 57 Lu to 58 Ce, there is a sudden contraction in atomic radius from 187 p.m. to 183 pm but the further decrease up to the last 4f element 71 Lu is comparatively low ( about 10p.m.)
 
						

							
Dear student 
As the atomic size increases, the basicity of oxides and hydroxides decreases. Due to lanthanide contraction, there is decrease in the size of lanthanide cations that increases the polarizing power of cations. That further increase the covalent character of the oxide and hydroxides.
Hope this helps 
Good Luck