Images indeed scale down, but they don't scale up to fill the available space.
But as I mentioned, when loading svg's you have two independent steps. The first step is the rasterization where you turn vector graphics into raster. There is a separate step of how you scale the resulting raster image. It makes sense that the second step is identical whether the source is an svg or png, but my claim is that the first step should be influenced by the desktop dpi.
I'll open a bug about it as Stefan suggested.
Regards,