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 [:D]

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.