Multiple physical effects are responsible for filtering. Each of them works best at certain particle diameters.
The combined efficiency of all effects happens to have a minimum at 0.3 microns. [1] This is called the "Most Penetrating Particle Size" (MPPS).
For very small particle sizes, filtering mostly works by diffusion (quoting from [2]):
>[Diffusion] is a result of the collision with gas molecules by the smallest particles, especially those below 0.1 μm in diameter. The small particles are effectively blown or bounced around and collide with the filter media fibers. This behavior is similar to Brownian motion and raises the probability that a particle will be stopped [...]
[1]: https://en.wikipedia.org/wiki/HEPA#/media/File:Filteration_C...