I might have time to look at this in more detail later, but a few brief comments.
- please share the reference of that first image. I want to make sure I understand what the context is and what they are trying to illustrate.
- In that picture it looks to me like there is some sort of optical distortion - that could be affecting the apparent relative sizes.
- I think if you point a camera perpendicular to a plane (image sensor plane and world plane are parallel) equal sized things on the world plane would project to equal sized things on the image plane, so any apparent difference would be human perception related, not projection / image formation related (unless lighting / shadows are at play.)
I’m not clear on what you are getting at with the “appear bigger even if they are equally sized” - but if you are saying that equally sized objects in a world plane project to different sizes in the (parallel to the world palne) image plane, I don’t think that’s correct and I think a simple “similar triangles visual proof” would clear it up.