When I use Inkscape to convert raster images I start with loading it with another image of the set, and then delete all image elements, to get a blank canvas of the desired size. Then I import the raster image as background, scale it to the same size as the set uses, and overlay it with my own drawing. This avoids size change.
The Magnetic SVGs I started from where not made by me, though. I copied them from the PyChess GitHub repository. And then did cut & paste jobs combined with some editing to create the compounds.
When I use Inkscape to convert raster images I start with loading it with another image of the set, and then delete all image elements, to get a blank canvas of the desired size. Then I import the raster image as background, scale it to the same size as the set uses, and overlay it with my own drawing. This avoids size change.
The Magnetic SVGs I started from where not made by me, though. I copied them from the PyChess GitHub repository. And then did cut & paste jobs combined with some editing to create the compounds.