Why do brighter stars/planets appear physically larger?
It is an optical illusion. Our brains associate brighter with bigger.  This is how software like stellarium are able to make convincing views of the night sky: they play this in reverse and use larger &quot;stars&quot; for brighter ones.  Your brain sees them the same.
Your brain is not tuned as a scientific device, there are lots of tricks that we can play on our visual system.