As the molecular weight increases, the number of atoms and hence electrons increase. Electrons randomly move around, not in even orbits as one might imagine. So at any time there could be (for example) 8 electrons on one side of an atom and 16 on another. This creates the temporary dipole-dipole or dispersion force. The more atoms/ larger molecular weight/more elctrons in a molecule, the more random movement of electrons to cause one side to be more negative and other positive = more temporary dipole-dipole's.
i hope this helps and doesn't come too late.