Looking at it from the ground, (I'm a volunteer firefighter), in my area, (near Romsey), a helicopter can dump 1500 L EXACTLY where it is required, wander over to a dam, and is back with another 1500 L in a matter of minutes. A bigger helo with 4 times as much water is 4 times as effective.
Small fixed wing delivers meaningful water more economically, but requires ground support at some sort of rudimentary airstrip, which is always going to be further away than the dam the helo is sluuuuping its next load out of. Maybe a couple of fire fighting tankers relaying water, or one big-ass tanker on site.
Bigger fixed wing would need to come here from further away, Essendon or Mangalore, and need the ground support, and probably not supply the fire with much more water than a number of smaller fixed wing, and nowhere near as much as a heap of helos.
On the other hand, a Martin Mars full of water on a going plantation/forest fire cannot be beat. It can turn an out of control fire I would run away from to something I can approach and black out almost instantly...