I'm not sure. Surface vs. volume is certainly and interesting thing that explains many scaling phenomena, but in this case, those huge radials had huge fins as

I'm not sure. Surface vs. volume is certainly and interesting thing that explains many scaling phenomena, but in this case, those huge radials had huge fins as well (correspondingly huger than fins on smaller engines in order to cool the much higher mass) to cool them under normal operations. So those same fins would equally rapidly cool the engine when descending quickly at idle power. So shock cooling is certainly an issue with big radials as well. Now I don't know the exact scaling relation, but I'd guess it's more similar to a smaller engine than a straightforward surface/volume scaling would imply. Or to put it another way, the surface area of the cooling fins is not proportional to the surface area of the cylinders without any fins, but to the power output of the engine.