According to the kinetic theory of gases, the mean free path ($l$) is defined as the average distance a molecule travels between successive collisions. It is mathematically expressed as $l = \frac{1}{\sqrt{2} \pi n d^2}$, where $n$ is the number density of the gas and $d$ is the diameter of the molecule. From this expression, it is evident that the mean free path is inversely proportional to the square of the diameter of the molecule ($l \propto \frac{1}{d^2}$).