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human
ELI5: How do turbocharged 4 cylinders get so much better gas mileage if they have to inject more fuel than an NA motor to keep the correct AFR?

It seems like most cars get better mileage because so many have a turbo 4 cylinder, but what difference does it make in fuel consumption if you have to use more fuel to match the higher amount of air in the combustion chamber?
gpt
A modern 4 cylinder turbo is probably around 2 liters. The equivalent would be something like a NA 3.6L V6 or even a NA 5L V8. 

The car is only on high boost during acceleration. So most of the time when cruising, it doesn't require a lot more fuel. The larger engines have to burn fuel for all that additional capacity all the time (albeit perhaps at lower revs) unless the engine has selective cylinder deactivation. 

The other savings is of course, weight.