I should have included this rough rendering in the first post. I just now drew it.
It’s over-simplified & quite exaggerated, but I hope it helps illustrate our discussion.
It shows: what the eye does + Parthenon’s architecture = resulting optical illusion.
Note that I show only the sagging, swelling, and tilting. There are many other “tricks” as people have noted – like the progressive swelling from column to column, their progressive tilting inward, the “bunching” of the columns at the end, etc.
The middle image is similar to what might appear on the neutral camera’s GG, in less-exaggerated form of course…