The more electrons a molecule contains, the higher its ability to become polar. Polarizability increases in the periodic table from the top of a group to the bottom and from right to left within periods. This is because the higher the molecular mass, the more electrons an atom has. With more electrons, the outer electrons are easily displaced because the inner electrons shield the nucleus' positive charge from the outer electrons which would normally keep them close to the nucleus.
This seems totally legitimate. However, (as chemwiki subtly points out) the main factor in work here is the volume occupied by electrons. Polarizability increase is mainly due to a volume increase 1. So, all you need to take into consideration is whether a molecule is bigger than the other in terms of comparing the stronger London dispersion force.
Simply put, the ability of a molecule to get polarized is called polarizability. See Wikipedia for more info.