Round motors give you a shorter crankshaft which doesn't weigh as much and doesn't require as many bearings. The biggest non radial air cooled engine I can think of is 720 ci, after that you add a radiator and coolant. So the BHP per pound works out better. R2800 was probably the best around, bigger than that got too complex. There was talk of making a turbo-compound 4360 for about 5000 bhp.