I guess I should ask exactly what year Mustang you have. Is it a Cobra? What mods does it currently have? 24 lb injectors were standard on some Cobra models. Did you put the 24's on yourself? What about the E cam? What heads are you using?

Is the mass air calibrated for 19's?

Edit -

I know it's a lot of questions - but here is my reasoning: ( I apologize if this is info you already know)

Your MAS determines the injector duty cycle by measuring how much voltage is required to heat a coil in the intake tract to a known temp. At the ait flows over the element, more voltage is required to heat it. At any given voltage, your car's computer references a fuel table and sends a signal to the injector to open for a certain amount of time. If the mass air is set up for 19lb injectors, the computer cycles the injectors based on 19lbs of flow. So lets say themass air tells the computer that it is taking 1 volt (This is an example) to heat the element to it's set level. The computer goes to the table and finds that at 1 volt, the airflow entering the combustion chamber needs 2cc's of fuel. With the 19lb injectors, it takes .05 seconds (duty cycle) to release that much fuel. Well, with 24 lb injectors, you are getting 4cc's of fuel.

In a mass air calibrated for larger injectors, it creates an artificial environment to make the computer think less air is getting into the combustion chamber than really is.

The beauty of a custom chip is that a good programer can alter the actual fuel tables and make the computer work instead of tricking it into delivering the proper fuel.