I don’t think it’s as clear any more that a small turbo engine will get better mileage/HP than a large displacement non-turbo engine.
For one thing, some of the large displacement engines have cylinder deactivation, which turns them into a small displacement engine when the power isn’t needed. Direct injection improves fuel distribution and allows running higher compression ratios. Turbo engines have to run lower compression ratios to keep from detonating. So when you’re running without your turbo spooled up, your engine isn’t as efficient as it would have been if it had been designed to be normally aspirated.
Plus, the newer transmissions have more gears, allowing the bigger engines to maintain a more optimum RPM for fuel economy.
I just looked up the fuel economy of a few sports cars, a couple with a small engine turbo, and a couple of normally aspirated, V6 and V8
Subaru WRX (4 cyl turbo, 2.5L, 230 HP) - Mileage: 19/25
Lancer Evolution (4 cyl turbo, 2L, 300 HP) - Mileage: 16/22
Cadillac CTS-AWD (6 cyl, 3.6 L, 304 HP) - Mileage: 17/26
Chevrollet Corvette( 8 cyl, 6.2L, 425 HP) - Mileage: 16/26
The 425 HP Corvette gets better gas mileage than the high-boost, 2L, 300 HP four banger. The 304HP Cadillac gets significantly better mileage than the 300HP Evo, despite having an engine almost twice its size and no turbo.
Both the high displacement engines get better gas mileage on the highway than do the small turbo cars - probably because the turbo’s working harder at highway speeds.
To make it worse, the turbo engines both need to run on premium. The normally aspirated engines run on regular.