I don't know about telling better the size from a picture. I can imagine seeing 2 pictures of the moon. One is extreme telephoto showing moon next to a building and it looks real big. Then there would be another image where moon is a tiny speckle in the sky. How big is the moon? I would rather understand a text: "its radius is x km".
I think the example is simplified to make its point efficiently, but also: the moon is something whose size would very likely be precisely explained in texts about it. While some hunting journals might brag about the weight of a lion that was killed, or whatever, most texts that I can recall reading about lions basically assumed you already know roughly how big a lion is; which indeed I learned from pictures as a pre-literate child.
A good, precise spec is better that a few pictures, sure; the random text content of whatever training set you can scrape together, perhaps not (?)
Reading "its radius is x km" would mean nothing to you if you'd never experienced spatial extent directly, whether that be visually or just by moving through space and existing in it. You'd need to do exactly what is being said in the paper, read about thousands of other roughly spherical objects and their radii. At some point, you'd get a decent sense of relative sizes.
On the other hand, if you ever simply see a meter stick, any statement that something measures a particular multiple or fraction of that you can already understand, without ever needing to learn the size of anything else.
