> The image should retain the same overall brightness as it is scaled down.
Incorrect.
While large regions of homogeneous color should indeed retain their brightness, thin one-dimensional structures are supposed to become darker and disappear gradually as the image is scaled down.
This is what happens when you move far away from an object, which is what "zoom-out" or "downscale" is all about. The total amount of light that you receive from an object decreases with the square of the distance.
It does. Imagine a one pixel wide vertical white line on black background. As you downscale it, by whatever reasonable means, the line becomes a darker and darker grey.
EDIT: This is in the context of the image shown in the article, which is a gradient image consisting in light lines over black background. Of course, if you have thin black lines on white background, they will become ligter upon downscaling. In practice, on a real image, some parts will become darker and others lighter, depending on their particular textures.
If you have an image consisting of a thin white line on a black background, then indeed as you scale it down the pixels covering that white line will become darker because each pixel corresponds to a small amount of white in a mostly-black square. (I am making some assumptions about what scaling is meant to mean here, but the conclusion doesn't change much if you think about it differently -- e.g., with pixels as point samples of a bandwidth-limited function.)
But those pixels are also larger and the ratio (amount of light in image) : (number of pixels in image) should not change.
The author of the OP is complaining that when you take an image and rescale it, its overall average brightness changes.
Incorrect.
While large regions of homogeneous color should indeed retain their brightness, thin one-dimensional structures are supposed to become darker and disappear gradually as the image is scaled down.