Hey Ryan, great question!

As the radius of an atom increases, valence electrons move further away from the inner positive core, thereby it would be easier to "peel off" those electrons as there is less positive charges to "hold" the electrons to its atom. This, like you mentioned, has to do with the electronegativity of atoms and their ability to hold on to electrons. Therefore, nucleophilicity increases going down a column. Its the same idea as going left of the rows. Atomic radii increases slightly going from right to left of the same row.

Hope that helps!

Cheers,

Molly