The fuel mixture in Closed Loop (normal driving) mode is controlled by the PCM using feedback from the O2 sensors. It does a good job of keeping it pretty lean.
Open Loop (Wide Open Throttle) is controlled by tables in the PCM. It looks at the mass of air that is entering the engine along with the RPM and calculates when and how long to open the injectors.
There is really a lot more to it than that, but that is the basic idea. The point is that the PCM is what sets the Air/Fuel ratio and the only way to make the engine run more lean is to change the data in the tables (that's what a chip does) or by fooling it to believe that less air is flowing in (modifying the MAF).
Using smaller injectors (eek) won't work, because the PCM is programmed to work around aging components to a degree. It would keep them open longer to keep the ratio where it should be.