It follows physics.
YOur mistake is to think of PSU as some time of Motor. FOr example your car. A 230hp car might be quieter at 130kph than a 90hp one because its reving not as high.
But we're not talking about that. We are talking about Electronics. There is a (physics) law that's called the law of the conservation of energy. It means that energy can only converted but never be lost!
The Term loss usually means phisical stuff that is not wanted but happens anyway.
For example: A PSU doesn't convert 100% of the incoming energy into the output energy. They only convert a part, in these days 90% or more are entirely possible in that process. The other 10% (depending on the Efficiency of the PSU) is converted to heat.
So we have two PSU, roughly the same efficiency at the same load (absolute, not relatively!) but the 550 and 850W use the same PCB, the same heatsinks.
How can the 850W be any quieter at the same load when they both generate the same amount of heat?
They do not. The "trick" is to use a higher RPM fan in the higher wattage units and just let it spin faster.
Since you usually can only go down to about 4V, wich isn't exactly linear, you come out at around 1000rpm for the 3000-3500rpm fan that is needed for the higher wattage units.
You can see that splendidly when looking at be quiet PSU because they openly write the max. RPM Onto the fan. and lower power units often get a 1500rpm or less fan and rotate it at 500rpm or less at lower loads.
The higher wattage get up to 2800rpm and also rotate the fan a bit higher at 100% load.
100% load, for the Manufacturer, means 100% load at the rated temperature - wich is 40-50°C for higher end units.
That is what the unit is designed/made for.
And why some units are unnecessarily loud at normal, up to 25°C room temperature...
Nope, its fairly normal that higher wattage PSU get a higher speed fan.
FOr example the Bitfenix Whisper M has a 0,45A fan for 450 and 550W, the 650-850W got a 0,6A fan.
And thus the idle RMP of the 450 and 550W Whisper M is somewhere around 450-500rpm, the 650W+ is at 700RPM.
WIth EVGA its even worse and they go straight up to 1000rpm for higher wattage models. I think I linked the 850W Leadex 2 for you.
EVGA will be the same, they just put an EVGA Label on it.