Ok, let me phrase that slightly differently; see if it makes more sense (I'm just getting my head around jet theory myself):
The piston engine can burn as much fuel as it can grab air - or to put it another way, there's no oxygen left in the exhaust. Power output is totally dependant on mass airflow.
By my understanding, the gas turbine is limited by temperature - you can't put in enough fuel to burn all the oxygen without melting the turbine section. So there's a bunch of oxygen left over in the exhaust (that's what afterburning/reheat uses). Therefore mass airflow doesn't limit your ability to turn fuel into power - temperature rise does.
That said, I think the penny just dropped.. I'm only considering the ability to generate power - the engine / gas generator, not the reaction mass - propellor / fan. The OP wasn't asking about that was he? How am I doing
