Hear is a prime example a 1:3,4 scale Gypsy Moth. Now the full size had a cruise speed of 85 mph, stall as low as 25 mph with engine on and landing speed 42 mph. That would mean by simplistic conversion you might expect this model to fly at 25 stall at 7 and land at 12 clearly not realistic if one likes to keep their models in one piece. Besides flown as shown in the video though not scale speed still looks very convincing.

Because physics, optics, and physiology (and math) govern the physical laws regarding the light being reflected off the plane, going through the convex lens of the eye, an forming an image on the 2D surface of the retina.

Because, as I also explained, how the brain interprets the information coming from the optic nerve depends on all the other images in same frame as the model, what that particular brain already "knows" about the object and environment (cognitive bias), and the heuristics developed by that individual over their life to speed mental processing. Two of those three vary greatly by person, thus different interpretations of the same image. In fact there's quite a lot of academic research on it ... I've appended a couple links on the subject of mental shortcuts ... heuristics, cognitive bias, and decision making (i.e. "Is that roll 'scale'?).

https://www.thecut.com/2017/01/kahne...illusions.html
grotzer_miller_lincoln2011pp27-49.pdf
https://corescholar.libraries.wright...ontext=etd_all
https://www.thoughtco.com/heuristics-psychology-4171769