As you have said its down to fluid dynamics or water friction. I happliy use 4mmID/6mmOD silicon hose its light and there is not much back pressure on the pump, But after 25- 30m the flow rate drops below 1 pint/0.6L You could then go to the bigger hose or a more powerful pump.
I use a bit less than 1/2L per min with a 28cm brush-the brush also has an influence as a 15" would need more water than a 10"