By default PD4ML convert all physical units to screen pixels using 72ppi, after that the entire document content is scaled depending on htmlWidth value (width of virtual HTML renderer frame) and on the chosen target paper format width: htmlWidth is mapped to pageWidth, so the actual scale factor varies and is not bound to 72 or 96 ppi.

If you prefer to work with physical units, trigger
pd4ml.protectPhysicalUnitDimensions() – in the case the elements should appear on paper in the originally desired size.