Lens diameter vs aperture

[-Tom-]

Looking at my Panny 20mm 1.7, the aperture ring is maybe 15mm, I won't go measure it, but it certainly isn't more than an inch no matter how you slice it.

On the other hand, some old primes my father has are slower than 1.7 but when wide open, it looks as if the entire 20mm Panny lens could go through their aperture ring.

Are there any correlations between aperture diameters and f-stop number?

[Fips]

Absolutely. The f-stop is defined as the focal length divided by the diameter of the aperture. Your dad's old primes are probably for a film camera, i.e. "full frame". A lens that would give a similar angle of view on this camera as your 20mm on a m4/3 would have an aperture diameter which is twice as large and thus the area would be four times as large.