An ecu in essence turns the injectors on and off at a specific duty cycle so it meets the fueling requirements. Thus the fuel delivery rate is based on injector duty cycle. It‘s all defined within the tune itself for the specific characteristics of a specific injector.
So changing injectors to something that deliver a differnt flow rate vs the duty cycle than what the ecu program expect will just cause issue. Bigger injector tors just throw too much fuel into the mix.
Put another way, your ecu may order the injector to operate 20% of the time which will result in a specific fuel delivery. If you just install larger injectors, then for the same 20% duty cycle, you may deliver 30% more fuel and it will run way to rich... it’s like putting a 4 barrel carbi from a 5 ltr V8 on a 2 ltr 4 cylinder engine. It just won’t run well and will cause issues.
Putting in lower flow rate injectors will result in a too lean mixture which can be catastrophic for an engine..
That‘s why you need a tune to correct the fuel delivery... but without associated mods, you’ll never use the injectors at a reasonable duty cycle so it’s a waste of time and money... Getting the correct injectors for the engine is the best approach forward.