My guess is it's some complex neurological response triggered by 'knowing' (or being sufficiently convinced) that one is surrounded by bright outdoor sunlight, rather than a physiological response powered by the light absorbed by the retina. What matters is the brain's estimate of the current lux value, which would take pupil size into account. This is just a guess.
1. It's not all about eyes. The skin also reacts to light.
2. Even though your pupil shrinks, you get a larger total number of light rays into the eye. Your visual cortex adjusts the image so you don't realize that a sunlit scene is a million times brighter than a lamplit scene. These extra rays may still affect something hormonal.
3. Even if it's partially psychosomatic or placebo, you would still be reinforcing the effect by telling yourself that you're doing it as a substitute.
