What follow is non a solution, just some considerations useful for further analysis.
It seems SH3D encodes the pattern for all the walls in a single PNG image when exporting to the SVG format (the PNG is embedded into the SVG). This means even if you have just 2 small walls, the farther they are, the larger the image will be. Now this shouldn't be a great problem, because PNG supports compression and the blank space between walls is prone to give high compression ratios.
The problem is the PNG image is produced without compression
In fact, I have achieved small file size even without resampling the PNG, just running an optimizer.
I will take a look to the code responsible for the exporting, but maybe Puybaret already know where and how to fix the issue.
