I also haven't driven there two cars, but in general smaller engines are better at running at low loads, like urban driving. If you do a lot of high speed highway driving, the bigger engine could match the consumption of a smaller one, since it can pull longer gears.
An extreme example of this was shown on TopGear, where a BMW M3 had lower fuel consumption than a Toyota Prius when following it around the TopGear test track (Prius driven flatout).
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