It has to do with the shorter distance the flame front has to travel. Those old large radial engines they had before the jets took over required 140 octane fuel, the compression ratio was not that high, in fact lower than many automobiles. But the large piston diameters meant that the flame front took longer to burn from the spark plugs and thus trapped more fuel to be compressed by the flame front. The same thing is happening with our model engines to a lesser degree. Because of this a very large engine of a given compression ratio will run best on lower nitro fuel, and a very small one will require high nitro to run best. It's not that obvious when you compare say a .40 engine to a .91 engine, but very obvious when comparing an .049 to a 2.0 sized engine.