The key distinction is between the physical iris opening and the effective aperture, usually described by the entrance pupil. In complex lenses, elements in front of and behind the diaphragm magnify or reduce how large that opening appears.

For exposure and f-number, the front-view apparent size matters because it determines how much of the incoming light gathered by the front of the lens can actually pass through the system. The rear view is related, but the amount of light available to form the image is set by what the lens accepts from the scene at the front.

For practical photography, almost everything is governed by effective aperture, not the literal mechanical opening size. That includes f-number behavior and diffraction limits. Diffraction is tied to the effective aperture; if only the physical hole mattered, lens designers could evade diffraction simply by using optical magnification around the iris, which they cannot.

So: the physical diaphragm diameter is mostly a design detail, while the effective aperture/entrance pupil is what matters for exposure and diffraction.