Downsizing the engine nearly *always* returns a fuel economy improvement, if all else is left the same. Sometimes the improvement is very small if a manufacturer regears, resulting in higher cruising RPM, but offhand I can't think of any vehicles sold today that get better economy with an optional larger displacement engine. This is why manufacturers today are downsizing their engines, in order to meet CAFE standards.
This is an example of a BSFC (brake specific fuel consumption) chart, taken from Wikipedia:
Smaller numbers (essentially) mean you're getting more power output for a given amount of fuel, or inversely, you're producing the same power using less fuel. Note how this engine (and nearly all engines) are most efficient under high load (going up the chart). Downsizing an engine results in higher loads at the same RPM under the same driving conditions.
You simply don't get better economy out of bigger engines in the same application, unless you're underpowered enough that driving in a straight line requires 3500+ RPM. Considering most passenger vehicles need less than 30HP (and many less than 20HP) to cruise on the highway...