As Jon has pointed out dark-adapted pupil size can vary hugely from person to person to say the least. So rather than making an assumption, that's probably wrong anyway, I'd suggest you measure your dark-adapted pupil size. It isn't difficult and there are several methods that a search on CN will show you. This is what I did.
I got dark-adapted for a few minutes to allow my pupils to dilate (there's no need for longer as full dark adaption is a chemical process). I then got my wife to take a flash picture whilst I held a ruler just above my eye. Make sure though you haven't got red eye reduction or pre-flash on.
Unfortunately my maximum pupil size is only 4.5mm (confirmed by my optician) - less than my wife's and way less than Jon Issacs'.
The issue with flash is that many modern flashes actually flash twice, very quickly. Since the retina appears red due to flash, you get the notorious red pupil look that you see with older pictures, double flash is designed to minimize that.
The first flash triggers the pupil shrink instinct. The second flash (after the pupil is constricted) takes the actual picture. This does a surprisingly good job of eliminating red eye in the picture. But if is unfortunate for measuring eye pupil size.
Modern phone cameras likely take a different approach, they likely use image processing to find the pupils and replace the red pupils with black ones. But the flash speed isn’t instantaneous, so the pupil still has time to constrict.
So using flash photography can mislead as far as measuring the eye’s pupil. And that doesn’t include the problems of prefocusing, as a dark room dark enough to fully dilate the pupil is too dark to autofocus, usually. Oh and let’s not forget perspective that can cause the measurement to be off by a millimeter or more, depending on how close the camera is and how far from the eye the ruler is held.
I bought a set of hex wrenches in metric format to measure my eyes. By holding them up and putting them between my eye and a bright star, if I can make the star disappear, the hex wrench is larger than my pupil. So I work down from 9mm until I can see the star (barely). With that method I determined my 46 year old eyes dilate to at least 7mm.