van dar Waal's forces

AL level:
When molecular size increases, the electron clouds become more diffused. This makes the electron clouds become unevenly distributed more readily, and thus instantaneously dipole and induced dipole are also formed more readily. The van der Waals' forces thus increase due to the increase in instantaneous dipole - induced dipole attractions.

CE level:
It is not required to know the explanation.

this force is the attraction of negative and positive between atoms in same kind of atoms and so more atoms more attraction.