why do more electrons increase dispersion forces?

Answer 1

More electrons usually means a larger size and that means higher dispersion forces in general.

Answer 2

why? or maybe the right question is how does the size contribut to the forces?

Answer 3

The larger an atom the farther the electrons are away from the nucleus; therefore, the easier it is to distort them. Here is a site that talks about that.
http://www.chem.purdue.edu/gchelp/liquids/disperse.html