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I want to try and say it more simply, lets see
The mass flow rate of an (combustion) engine is calculated by comparing pressure, volume, and temperature to get some rate of airflow in mass per unit time which closely correlates with engine power.
If you ask "pressure from where?" I will answer that the mass flow of air into the turbo has to equal the mass flow out of the exhaust port, just like with current into an electrical circuit, so what we really have is a set of partial differential equations which represent the pressure at EVERY POSSIBLE MOLECULE In the entire engine, easily calculable but spread apart by boundary conditions.
Luckily I don't need to write or solve any math to apply our universal law that mass in has to equal mass out. If we do something, anything, to the exhaust which will influence one of those major variables: static and 1st 2nd 3rd differentials of pressure, temp, & volume then the mass flow will also change, perhaps in a number of non-linear ways.
By making the assumption that larger diameter is going to increase flow rate we are essentially saying that engine power is increasing, which implies the turbocharger shaft is speeding up, because there is no way to get any additional mass flow rate through a turbocharged engine without altering the temperature of the air incoming to the turbo, or adjusting the pressure of the air entering a turbo (elevation is the only way to do that unless you are compound turbo/boosting which is way off discussion topic), or changing the volume of air pumped by that turbo which is only possible via shaft speed. The critical idea here being that turbochargers always flow a specific volume number at some specific shaft speed no matter what the temperature or pressure is, in classical physics.
For this to be a fair comparison of stock downpipe vs upgraded downpipe, I feel that shaft speeds (and turbo inlet air temperature) should be identical. You may not feel that way and so you are also right in your own way if so.
I suggest that we bring shaft speed back to the original shaft speed by adjusting the wastegate properly, and the boost will become lower than it was before, due to the larger downpipe and WHAM you've just solved the boost creeping issue with a larger downpipe upgrade because turbine mass flow rate will be the same as it used to be with the stock downpipe (engine will make the same power) but pressure in the downpipe will be lower because of the larger downpipe, and the gate will have all that extra downpipe volume and reduced pressure to eject exhaust gas into.
Hope it starting to make sense !
okay I see now how to say it with one sentence:
"Install larger downpipe, reduce boost pressure, engine will make the same power as before but boost won't creep"
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