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The real reason automakers are downsizing engines is to reduce costs. All the extra material costs money, the additional machining of a larger engine costs money, the cost to ship the engine is more per unit, the more complicated fitment of a larger engine is more costly, etc.
Fuel economy is typically not the primary concern when it comes to engine selection/design. Maybe not the most relevant example, but both generations of chevy volt were just ordinary i4's. Toyota actually upsized their engines to take advantage of the Atkinson cycle, Ford has been using the same 5.0 and EcoBoost v6's for many years now, honda has kept the same long runs of its 4cyl's and 3.5 v6 with exception of their new line of turbo i4's.
Summary: engine size is nearly irrelevant for FE, so many other factors go into the equation.
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