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Back to the economy thing...
One major factor in an engine's BSFC numbers is "piston ring swept area". As this swept area is 1) a major source, likely the biggest, source of friction, 2) an area for piston to cylinder heat transfer (wasting combustion heat) and engines with less swept area also have less surface area in the combustion area to transfer heat, boosting efficiency.
Engines with larger displacement spinning slower have significantly less piston ring swept area.
It's no surprise the large displacement V8 engine can achieve impressive economy in a lightweight, aerodynamic car. All it takes is proper tuning and proper gearing.
However, at idle, larger engines consume more fuel. And around town, this takes it's toll.
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