Why does a smaller aperture size have a high depth of field? What is the physics behind this?
Why does a smaller aperture size have a low depth of field?
It doesn't; you've got it backward. Decreasing aperture size increases depth of field.
What might be confusing you is that smaller f-numbers indicate larger apertures. For example, an aperture setting of f/2 indicates a larger aperture than does f/2.8.