Note that I wrote most of the below before Brad's post, so some of it is a bit redundant. I left my computer for awhile before finishing it . . .
Let me see if I can throw down a little 101.
A Ford MAF sensor measures the mass of air flowing into the engine over time. Hence the units being either Kg/hr or lbm/hr. Nothing more. The MAF does not care about the size of the injectors, or any other component of the engine. Much like an Engine coolant temp sensor doesn't care what size your injectors are.
An accurate MAF reading is only guaranteed when the MAF is used in the full intake tract it was originally calibrated with, ie the stock stuff. If you change anything from the air filter to the throttle body, the transfer function will no longer be accurately calibrated. A good tuner can compensate and "fix" the transfer function properly in the pcm, but that is a different subject.
In an effort to sell more parts, the aftermarket MAF companies have started tweaking thier trasnfer functions so that they will more or less function w/ different than stock injector sizes without changing the pcm. They do it in the same method that brad described above, by offsetting the transfer function by the same % difference as the new injectors are from the stock ones. The PCM now thinks far less air is entering the engine, and therefore far less fuel is asked for. Because the injectors are larger now, the total amount of fuel almost balances out. ALMOST. Aftermarket MAFs are notorious for inaccuracy and poor quality control, so the stock trasfer function is rarely accurate. Further, because the PCM is thinking there is far less air entering the engine than there really is, the PCM can never do the load calculations properly (which is a major factor in determining timing).
In conclusion, using a MAF meter, or MAF signal modifier to compensate for different size injectors is a HACK mod, and prone to poor results in overall performance (usually driveability suffers, as does low rpm performance).
