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 dar

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.