How the presence of d-orbitals affect size?

[johncena]

In my textbook , it is given that Atomic radius of Ga is smaller than that of Al due to presence of d-orbitals in Ga.
But how does the presence of d-orbitals reduces size?

 

[Char. Limit]

But it's not...

The atomic radius of Aluminum is 125 pm. The atomic radius of Gallium is 135 pm.

 

[johncena]

No, i refered wikipedia article...Atomic radius of aluminium is 143 pm

 

[Char. Limit]

http://www.chemicool.com/elements/aluminum.html
http://www.chemicool.com/elements/gallium.html

This website disagrees.

 

[johncena]

Which website is true ?I am really confused .