If width is not explicitly set for images in the documentation, an implicit width of 80% page width is imposed for the PDF version (see doc/external/extensions/pdfimgsize.py), which will cause images with low aspect ratio to potentially cover several pages and may cause a crash.
An updated solution to this conundrum has been introduced in commit r39236. Since there seems to be an inconsistency in DPI assumptions, as in 72 vs 96, we apply an implicit scale of 3/4 on all SVGs lacking explicit width/height/scale. Looking at the function size_for_node() in https://github.com/rst2pdf/rst2pdf/blob/main/rst2pdf/image.py, it is not obvious why large SVGs that are scaled down prior to our 3/4 factor, are still rendered above 75% width. However, both large and smaller images appear fine in the resulting document, so this is sufficient for now.
