>Even e.g. a cargo 747 has ~750 m3 (=~200k gal h2o) of cargo space, but 112000 kg (=~30k gal h2o) of payload, so one could only fill ~1/6 of the cargo volume with water.

yep. Yet they still have to push through the air and have the mass and the cost of the plane for that unnecessary 6x volume. That is because the plane is retrofitted instead of having been built for the purpose of carrying high density payload. (also that 6x larger than needed volume makes those retrofitted planes much more susceptible to the high wind gusts, etc. which almost always an issue with those fires)

CA has C-130 water tankers with 20 ton payload (16 ton is the water itself). Total capacity of C-130 when used as fuel tanker - ie. when its own fuel and payload combined - is 30 ton. Empty it is 35 ton, max - 70 tons, and cost $60M+.

Taking a real cheap large drone as an example to have real base numbers to work with - German V1 (pulsejet doesn't matter, the piston engine of course would be the way to go here as we don't need jet speed) - 2.2 ton total weight with 0.5 ton fuel and 0.85 ton payload. The ratio of fuel plus payload to weight is already better than C-130's (at that ratio the C-130 would have to carry 35 ton payload instead of its current 20 and 20 ton fuel instead of its current 10), and with piston engine of the same weight as the pulsejet we'd get even larger thrust - i.e can get even higher payload ratio - while using much less fuel (the pulsejet had atrocious efficiency).

So, it would take 15 drones of that size for one C-130. These drones are much simpler and easier to build and thus cheaper than even small Cesna, thus 15 of them would be at least ~10x cheaper than one C-130.

Dropping 15 loads of 1+ ton water instead of one 16 ton load seems to be better in most cases as the fire line isn't a straight line, and the large plane like C-130 has to maneuver, etc. while flying low over the fire and being subjected to the fire draft, wind gusts, etc.

The 15 drones can drop those 15 loads of 1 ton using dive-bomber style - impossible for C-130 or any other air tanker currently in use - thus avoiding that prolonged horizontal low-flying over fire and thus having much less risk/danger form the wind gusts, etc.