One of the many forms of interaction between molecules is the London dispersion force. London dispersion forces are a kind of van der Waals force as they arise due to dipoles that are created in molecules. When electrons circle around atoms their concentration is not the same throughout the sphere they are revolving in. It changes repeatedly, with one area having a higher negative charge and the other having a higher positive charge. These charges attract each other.
London dispersion forces are weak as the dipoles created due to the change in charge does not have too much of an effect. This force is increased by an increase in the size of the molecule and the presence of more electrons in the molecule. So London dispersion forces are stronger between molecules that have more number of atoms and the atoms themselves are large which increases the atoms that they have.