What he said above. ^^^^^^^^^^^^^
A 1.9 TDI has a swept volume of 474 cc per cylinder. To achieve a geometric compression ratio of 19.5:1, it would have to have a total clearance volume at TDC of 25.62 cc. This volume includes the bowl, valve pockets, ring land volume, squish volume... EVERYTHING. The only way to accurately measure this is to cc the combustion chamber. The most common way to do this is by titration of a liquid (usually thick motor oil) directly to the engine.
Within the parameter of compression ratio, there is another parameter particularly applicable to DI-Diesel engines known in engine development circles as the "K-Factor". This is defined as the ratio of the piston bowl volume alone to the total clearance volume. The consensus is that a high K-Factor is desirable (large percentage of the clearance volume belongs to the bowl).
To reduce the compression ratio to, say, 16.5:1, you need a total clearance volume of 30.58 cc if the swept volume (bore x stroke) remains the same. It's obvious that if this all of the increase in the clearance volume comes from the bowl, the K-Factor increases (this is good).
How did I determine where and how much to remove material from the bowl? By visualizing the angle of the spray coming into the bowl from the injector nozzle and imagining a flow field in my mind, and not wanting to spray against any walls but rather in an area of high flow velocity (even though we don't have the benefit of performing CFD, it helps to have seen countless CFD visualizations of in-cylinder flow fields for a DI-Diesel piston bowl to get a rough idea).