That 5.5L 670HP engine is impressive, for sure. However, it is not compact, simple, inexpensive or easy to produce. Nor is is particularly efficient with a operational windows of BSFC hovering around 0.55 Lb fuel per HP hour.
In very general terms, larger, fewer and slower pistons create less piston ring swept area, less combustion surface area (by displacement/air moved) and a more efficient engine due to lower friction (ring swept area) and surface area exposed to heat. This is why very large engines generally have better BSFC numbers.
Ever wonder why piston powered airplanes still use 1930's technology? Because these lightweight engines have BSFC of 0.38 to 0.42, that's why. Remember, we fly by weight and need range.
