Yeah from what i have read of late, way too small! Seems that maximum power is obtained at a duty cycle of between 60-70% & no more! Therefore, as an example, if you calculated that your engine requires 600cc of fuel flow per injector, then you should use 1000cc injectors, to leave plenty of head room on the duty cycle and not over work the injectors.
I have been reading "Forced Induction Performance tuning" by Graham Bell, he goes into great depth with calculations for all this stuff.
I have also been talking with a guy that runs a tuning shop here in NZ, he is a computer expert etc and has a RB30DET powered R32 skyline racecar, which is making 400kw at the rear wheels, he is using 1000cc injectors.
Stony what size inj's have you replaced the 720's with?