By sacrificing emissions.
You can get more power per stroke if you don’t care about emissions, and that can get you a better fuel economy, but you may fail your local e-check if it’s tuned.

Volkswagen got in trouble for designing their cars to change the engine tuning when it detected it was connected to a testing rig.

Factory tuning is done the way it is for fuel compatibility, engine and component longevity, and a wide range of consistent performance at any reasonable altitude.

Changing the timing of the ignition, altered valve timing, and changing to a higher octane fuel can give you more power and better crusing economy at the cost of longevity, not being able to use a lower octane fuel, and possible issues if you drive at an altitude far different than the one it was tuned at.

Power and fuel economy are not the only two variables. Emissions are the biggest thing that comes to mind: your new high-performance remap might put your car outside of the emissions limits for your market.

Some aggressive remaps can also adversely affect how the car drives in normal use. Think rough idle, or jerky acceleration. The vast majority of drivers do not want that.

If something promises to increase power and economy at the same time I assume it’s bullshit.

With that said you can increase one or the other with remapping depending on your focus.