Currently, the only way to control how the SVG is scaled on the chosen page format is to change the margin setting. This works well in many cases and is a nice default behaviour, but is inconvenient for cases where careful scaling has been done beforehand. Here are a few examples I can think of:

• plots with filled polygons where hatching pitch must be precisely controlled
• technical plots where final, physical scale is important
• small plots in a specific place on the page
• etc.