It's basic physics. If by "good" you mean "simple," then you probably never had a good explanation because the why isn't simple.
Start at that link then at the bottom of that page are links to the math and deeper discussion about why the why is the why
https://luminous-landscape.com/under...s-diffraction/
The fast and furious explanation is that the edge is
always messing with the light and it becomes proportionately more of the image as the central area inset from the edge becomes smaller in proportion to how much edge there is. i.e. less clean central light versus more messy edge light the smaller your aperture.
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