The mean free path is an average distance over which a moving particle substantially changes its direction or energy, typically as a result of one or more successive collisions with other particles.

The mean free path for a gas molecule depends upon the diameter, $d$ of the molecule as: $\lambda =\frac{1}{\sqrt{2}\pi d^{2}n}\propto \frac{1}{d^2}$,

$\lambda =$ the mean free path
$d=$ the effective diameter of the molecule
$n=$the number density of molecules