How much it can haul is also relative to the operating conditions, both at the reload base, and the drop site. There are a lot of factors to consider.
The 747, in theory, was a great idea. The cost per gallon of retardant delivered to the fire was high. The turn-around times were significant. The number of reload bases was small; not many bases or fields could handle that aircraft or were set up for it, nor were reload pits stressed for it, or in many cases, big enough for it. That meant longer times to and from the fire. It meant higher drop heights, faster drop runs, and often a lot more retardant than was needed tactically for the drop or the fire. A swept-wing turbojet also offers significant disadvantages in a turbulent low-altitude environment at slower speeds, especially in smoke, especially populated with numerous other aircraft operating, in mountainous terrain.
Similarly, other types of aircraft have their own limitations. The most numerous tankers out there presently are SEATs (single engine air tankers); almost exclusively Air Tractor AT-802's. These 800 gallon tankers can use small airfields, can operate closer to the fire, have very short turn-around times, are maneuverable and designed for retardant delivery, but also have a government-mandated restriction of 30 knots wind over the fire (not a bad thing for anyone operating close to terrain in the mountains, in a fire). Strong crosswinds are also a concern at the reload location, and obviously they're not as fast as a P3 or 747. Each aircraft has it's own operating limitations with respect to the job, not the least of which are stresses induced on the airframe by repeated exposure to mountain firefighting conditions.