Thread title asks it all. I would really like to know this.
Because they subtend a smaller angle at the eye.
If you take two similar sized objects, A and B, whose top and bottom can respectively be designated as At, Ab, Bt, and Bb, and place B further away from your eye (which is at E), and A closer, then the angle At-E-Ab is going to be larger than the angle Bt-E-Bb.
That may not be very clear as written, but the point is hardly rocket science. It could be made much clearer with a simple diagram, but Google did not find me a suitable one. If you can translate my description into a diagram you will probably be able to see the point.
If your question is really “Why are we sometimes deceived into thinking that objects further away really are smaller than they truly are?”, then the answer is much more complicated. But, in most cases, we are not deceived.
It’s called perspective, and IMO you really shouldn’t need to ask this question if your age has more than one digit. Perspective (graphical) - Wikipedia
Because, thanks to the structure of the eye and the fact that light travels in straight lines, the image of a more-distant object actually falls on less of your retina. It’s all about the Visual Angle.
You guys are mean! 
I though it was a great question. Maybe it’s more a question of, “how do our brains learn to associate small with distance?”. As the father of a 4-month-old, I can surmise that a lot of it is just trial and error – reaching out and touching something (or missing it.). (See, too, the descriptions of people recently cured of blindness, e.g. in Annie Dillard’s book Pilgrim at Tinker Creek.)
Because if everything looked the same actual size as close-up you’d have some difficulty in moving about.
I am well aware of perspective. Thank you for your input.
This is more of what I was looking for.
Objects that (appear to) change their size depending on distance just all of a sudden seems odd to me.
This is good, so distance and the size of objects is a product of the human (and other critters) brain/eyes?
I’ve heard that people who have lived their whole lives in a jungle never develop a sense of perspective. Is that true? It seems a little urban legendy to me.
Not really, no. A camera will show the same effect.
[Moderator Note]
Hairy Bob, there’s also really no need to post insulting replies in GQ. No warning issued, but don’t do this again.
Colibri
General Questions Moderator
The way our eyes are formed and the way our brains interpret the data certainly have an effect upon the way things are perceived, often in surprising ways, but at the root of things is the very first answer you got from njtt. The sun is huge, at 1,400,000 km diameter, but it’s 1.5 X qo[sup]8[/sup] km away, so it only subtends about half a degree. No matter how your eyes are arranged, unless you have a telescope in there, the sun’s not going to look any bigger. Angular subtense really does determine how large an object appears to our eyes.
Associating size with distance (and learning about parallax) is something else – but it doesn’t make the sun appear any bigger or smaller. Being low on the horizon gives the impression of the sun being bigger, but if you use a ruler or any sort of comparator you can definitely show that it doesn’t really appear any larger. Tricks such as different shaped pupils, or games with aperture stops (such as using a telecentric lens) or use of zoom lenses may make the object appear different relative to its background, or make it seem as if you are seeing it in a different way, or make it appear to move back and forth. But none of these change the angular subtense of the object, and none of them will make it any larger or smaller.
But the camera is very similar to our own eye? Lens, shutter, film = cronea , iris, rods and cones?
Correct – but there’s no brain. tdn is objecting to your suggesting that the brain has something to do with it.
Got it. Kinda mostly.
We have a stereoscopic vision (both eyes are situated in front, contrarily to, say, horses who have an eye on each side), so the brain receive visual input about an object coming from two slightly different angles. That’s what allows us to have a sense of perspective/depth perception. People who are blind in one eye may have difficulties with perspective, especially for objects situated at a close distance (for objects situated at a long distance, AFAIK, it’s more a matter of training, for instance you know what size a house is, so you’re guessing that a very small house is actually long away. The angle difference between the eyes becoming unoticeable for remote objects).
Er. no, I said nothing about brains. I was saying it is a product of the geometry of the situation.
However, if your question was intended to be more along the lines that JKellyMap suggested - why do things look the way they do? - then we would indeed have to start bringing brains into it, and the answer would be both complicated and scientifically controversial. Philosophers and scientists have been arguing with one another about these issues for centuries, and they have not reached a settled conclusion.
While it’s true that a camera works in much the same way as an eye, anything which provides a sense that could even remotely be considered “vision” would have to work the same way.
Stereo vision certainly has value, but parallax - the way foreground objects move against the background - is probably more significant unless the eye is stationary.
This isn’t an answer, anyway: It’s a word noise unless you go on to define perspective in a way that doesn’t just apply a different word noise to the same phenomenon.