You
can increase fuel efficiency by turboing an engine of the same size. The gains will definitly not be as large as going to a smaller engine, but there are gains to be had. It may not be worth the money of course.
For example, lets use this BSFC chart from a 1.9L Saturn engine.
With a turbocharger, you could vastly increase low end torque and widen the peak BSFC area to much lower rpms. This would necessitate a different type of tuning system than what is conventionally used on turbocharged engines. The system would aim for peak torque while keeping the engine in closed loop always. This would require a way to dynamically alter boost pressure based on rpm and load (hey, I never said it would be easy). This would allow you to always have optimal cylinder pressures. The only variable left really would be RPM (more specifically piston speed).