What optical phenomenon causes a very bright star to visually appear larger than a dimmer neighbor?

Answer

The Airy Disc Effect.

Light from bright stars floods the pupil, spreading into a pattern called the Airy disc on the retina, causing the brain to interpret the signal spread over more photoreceptors as a physically larger object.

What optical phenomenon causes a very bright star to visually appear larger than a dimmer neighbor?

#Videos

STAR BRIGHTNESS EXPLAINED - YouTube

What Determines the Brightness & Size of Stars? - YouTube

astronomystarbrightnessLuminosity