Ahh, now Turbo engines aren't greatly more efficient than N/A engines in terms of turning fuel into power when running flat out, in fact the rich fuel mixture they like often makes them worse.
The reason for running a tiny turbo motor like a 1ltr eco boost is as follows.
If you wanted as much power as a 1ltr eco boost you'd usually need a 1.6, when you were cruising at 100kph you might only be using 5kw of power, and to get that small amount of power out of a 1.6 you'd need the throttle almost closed, the 1.6 would be spending a lot of energy trying to suck air past a closed throttle.
The 1ltr, because it has less capacity, and less power (when not on boost) could cruise at a higher throttle setting, working itself harder, spending less energy trying to suck air, and working more efficiently because of it. Then when you DID need to accelerate hard, the ecoboost could just spool up the turbo to make the power that it needed.
The reason you can't see this in the engine designer is that it only shows how effecient the engine is at turning fuel into power at full throttle, once it's in a car you'll be able to find out what kind of economy it actually gets, taking account of gearing, how much power it needs to cruise at 100kph, and what throttle setting and RPM that puts the engine at
