The primary philosophical thrust against "pictorialism" was this notion that photographs should look like photographs, and not like paintings. What a photograph should look like was left as an exercise to the reader.
Edward Weston wrote on this subject, on the properties of the photograph as opposed to the painting:
... the physical quality of things can be rendered with utmost exactness: stone is hard, bark is rough, flesh is alive, or they can be made harder, rougher, or more alive if desired. In a word, let us have photographic beauty!
His thesis is, as I understand it, that the whole point of the camera is an exactness, a geometric truth, a fullness of detail. This is a theme that, in truth, was belabored by many a pictorialist.
Lots of people have spent a lot of words on how eyes work, and how that is relevant to photographs and photography. See, for instance, the unsinkable Ming Thein with the usual rot about rods and cones and how the eye only sees clearly in the middle.
All pointless drivel. Vision is a construct of the brain. You'd think that the eye would be relevant at least as the source of raw material for the visual cortex, right? No. Your brain will cheerfully paint in made-up detail and information. Studying the eye is not pointless, but it invariably misses the point. I wrote a bit about some related ideas over here in this post, some time ago.
What is true is that when you're looking at the real world, what you're actually seeing is a very very small area where your attention happens to be focused. This is usually, but not always, in the center of the field of vision. When you look at a landscape you see the peak of the mountain now, and a moment later you see the curve of the river, and still later the color of the wooded slope. You don't see them all at once and, in fact, if the mountain were to vanish while you were looking at the river you would not notice. It's possible your visual cortex would edit your memory to give you the illusion that you'd noticed (see cronostasis illusion) but you would not notice. You don't see it.
Most of what you "see" when you're looking at the world is invented material, painted in rather roughly by your visual cortex, based on memories of what was there a moment ago when your attention was there rather than here. Emerson, interestingly, was fully aware of this and advocated emulating it with the use of selective focus, to isolate the single important thing in the frame in much the same way your brain isolates whatever it is that your attention is focused on. He had the science wrong, but the idea was solid.
But he missed what is arguably the point of photography (or at any rate, a point of photography).
What photography does is two things:
First, it folds up a bunch of the world into a much smaller portion of the visual field. Now you can and do see the mountain, the river, and the wooded slope all at once. This, I think, is what Weston is talking about. You can actually see the textures, the details, all the little facets of the scene, all at once. You're not relying on your visual cortex to "paint in" a bunch of stuff, you're not relying on your unreliable memory to "fill in" the stuff around the edges that you're not actually seeing at this instant. It's all right there.
Second, it encapsulates the folded up part of the world into a single object, a photograph, that you can look at and appreciate as a single thing. No longer do you have a mountain, and a river, and a wooded slope, all separate objects, all at different distances, in different places, with different light. You have a single object which you can apprehend all at once.
This adds up to presenting a slice of the world in a way that allows us, in a sense, a far more direct experience of it. Of course we're removed from the world, because it's a picture and not the thing itself. Simultaneously, though, we're closer to the world because so much has been compressed into a single visual unit, digestibly proportioned. We see the mountain, the wooded slope, the river, all at once.
A small painting is also capable of being seen all at once in the same way, but not being a packaged up slice of reality, it is not at all the same thing.
This, I think, argues for small print sizes. In order to fully realize the power of the photograph, we should print small. We should not attempt to create prints dimensioned to appear as a window, so that from the expected viewing distance the things in the print appear proportioned as they did in reality. Instead we should print small, to pull that view in tighter, to allow the seeing of the scene, the subject, whatever it is, to proceed in this different way.
If you're not going to do that, might as well paint!