Well in your example, the 2.0 is a 4 cylinder engine and the 3.0 is a 6 cylinder engine. More pistons means more displacement, which means more air getting sucked into the engine for each revolution of the crankshaft, which also requires more fuel to create the proper air/fuel ratio. All of that generally means more power, and lower fuel economy. Basically, a bigger engine will make more power, over a more usable RPM range, while using more fuel.
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