I realised after that last post I could have explained the camera thing better, and while I'm at it there is probably a lot about our conversation that could do with spelling out.
Binastro is the astronomer and I'm a birder and I'm sure to make mistakes, but I hope he will forgive an amateur and correct me tactfully.
I'm going to skip planets and talk about stars for the moment. I'm sure we've all looked at the sky and checked out the big ones and then looked for patterns in the small ones. It rather surprised me to realise in fact they are all absolutely minuscule. With the biggest telescope any of us are likely to afford they would still make a smaller angle to the eye than a single receptor on the retina. What we've been discussing is why we don't see this and why the stars and the planets like Venus can look hugely bigger than they really are. They look bigger the brighter they are for a number of distinct reasons, but I'll only mention a few that seem from my reading to be the most important.
As soon as light passes through the atmosphere and glass there is a spreading of the profile of the light due to diffraction and to some extent scatter. Most of the light stays in a peak but there is a boundary to the spot distribution that spreads out from the centre. This is progressively more obvious in a photo with over exposure as that will boost the intensity of this boundary and that alone will make brighter stars look bigger. A diffraction spread also occurs in the lens of the eye. Any additional optical aberrations in the eye or the optics will again increase the spread further due to blur. Unfortunately the rest of the eye has a bunch of imperfections as well that increase the scatter further, and those imperfections accumulate as we get older.
In fact the eye has similar problems with overexposure as a camera. The technicalities are different, but the eye has comparable metering properties to a camera and tends to average a scene leading to over exposure very bright spots.
There is one more important factor to consider that can be particularly significant to the astronomer dealing with very high contrast light sources. Once the light hits the camera sensor or more particularly the retina of the eye it isn't all absorbed. Some of it is scattered across adjacent pixels or retinal photo receptors and it is this what I understand is often more important than the other spreading functions in making bright stars look bigger to the eye.
The eye is reported to produce an image over a trillion fold range of light intensity, but it only does so with a very small range at a time. We can only discriminate proximately 250 shades of grey (I'm sure there must be a joke there!). You're computer screen or your photographic printer usually works on a 256 scale as does an very basic camera. The metering the ISO, f-number and exposure move that 256 range up and down some of that trillion fold range to hopefully produce an image to match the eye. A high quality camera will capture a luminance range way beyond anything the eye can manage. At 16 bits it will produces digitised information 256 times the range the eye can deal with in one go and the very best cameras will do more than that. If that information is compressed into a single 256 scale image the brightest stars will look disproportionally larger due to over exposure and the feint ones will look smaller, due to under exposure, not unlike the eye, but in fact the captured data can reveal that all the stars have in fact the same apparent diameter; if not the true one. The eye doesn't have this scaling capability and as a result differences in brightness means they always means appearing to be different sizes.
I hope I got most of that right.
David