"Those turbocharged Only if you drive them like in the lab tests. If you drive them normally, expect a very big discrepancy between the lab tests and real driving mileage. Much bigger than for normal cars with big engines.
The whole reasoning behind downsizing goes like this:
1. small engines are lighter than big engines, so they offer better mileage in city driving because there is less mass to accelerate
2. small engines have lower internal friction than big engines, and they need less fuel when idling, or near-idling, which you often do in city conditions
3. most of the time, most drivers don't need much power, particularly for city driving or when cruising at a constant speed
This leads to a conclusion that to maximize fuel efficiency, you need a small, low-powered engine for daily use with a strong turbocharger to help get more power in very rare events when you need it (like passing over).
However, there are a few caveats with this line of reasoning, and I guess some (Japanese) car manufacturers already noticed it and refuse to put downsized engines into their cars:
1. A turbocharged engine is much less efficient at full throttle than an NA engine of the same power (couple of reasons for this: lower compression-ratio, higher lambda to cool down a very small engine so it doesn't melt, losses of energy in the compressor / turbocharger).
2. If you downsize too much, normal driving will require to run the engine near the top of its power and make use of turbocharging very often.
3. The longevity of the engine is negatively affected and there are more components that can break. And modern designs that tend to reduce turbo-lag are complex and expensive to repair (dual compressor/turbocharger designs, variable geometry turbochargers).