Not arguing with Poly at all here, he cites a very specific case, but IN GENERAL:

Flame travel for a quiescent mixture in a gasoline engine is woefully slow. As the rpm and stroke length increase, the flame travel is even less than the piston speed! That's why squish is so very important, it mixes the flame through the chamber which speeds up the lighting off process.
Because the flame travel is slow, larger diameters give longer time for the flame to travel and give more time for the mixtures out at the ends of the flame path time to form knock-prone compounds.
As a result, with everything else being equal, larger diameter cylinders need more octane.

R.