It makes sense that a larger turbine on a turbo engine would increase the noise level. However, instead of being dependent on the actual size of the turbine, it’s dependent on where the turbine slider is set. This is a problem because the size of the turbine changes depending on the size of the compressor.
So you can have a tiny turbine with a tiny compressor, or a huge turbine with an equally huge compressor, and they’ll have the same noise level–because the turbine-size slider is in the same spot.