Why do brighter stars/planets appear physically larger? Look at Sirius and then some 2nd magnitude star. Sirius appears physically larger despite both stars subtending approximately the same angular diameter.
Callisto is far enough from Jupiter at its greatest elongation to be seen with the naked-eye, but a common reason it still isn't visible is because Jupiter's glare hides it.
To be clear, I'm not talking about Jupiter's additional reflected light making it harder to spot dim Callisto the same way the Moon makes nearby stars harder to see. Jupiter literally appears larger in the sky because of how bright it is. Why is that?
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.