I don't see it as very mysterious..? I think it's fairly well established.
If the room sound doesn't correspond to the direct sound, the illusion is ruined. Non-uniform dispersion against frequency is a major discrepancy between direct sound and room sound.
A wider baffle automatically helps to make the dispersion more uniform because it lowers the frequency at which the baffle is effectively preventing the wavefronts from the drivers from 'going round the back' of the speaker and instead keeps them projecting forwards.
Mr. Grimm goes into it in great detail when talking about his LS1 (a 'small' speaker with a wide baffle):
https://www.grimmaudio.com/site/assets/files/1088/speakers.pdf
Mr. Putzeys talks about how his active cardioid cancellation system creates the effect of a wide baflle
https://www.kiiaudio.com/acoustics.php
Mr. R.E. Greene talks about his exasperation over the fashion for narrow-fronted speakers here:
http://www.regonaudio.com/Audio in Modern Times.pdf
And the 8c uses special holes in the box to help keep the dispersion more uniform using passive acoustic means.