Changing the effective size of an engine is tricky. This is especially true of a diesel which requires a constant compression ratio. The way it is done here is to vary the boost. There must be just the right amount of air inside the combustion chamber at the start of the compression stroke. The calculations below were done for generation 9. There have been some improvements since then but the basic process is the same. At maximum power, the combustion chamber is biggest. Additionally, the boost is greatest. The opposite happens at low power.
Improvements since generation 9 are:
1) The combustion chamber is shaped to minimize its volume at low power. The effective size now varies by a factor of four.
2) The throttle has an indented area. That area is aligned with the inlet port only at low power. This allows selective charging at low power.
3) The clearance volume of the supercharger is increased. This increases its effective capacity.
The calculations will have to be redone but they are still reasonable close.