in rsvg-size-callback.c:
 ...
     if (real_data->keep_aspect_ratio) {
         int out_min = MIN (*width, *height);

         if (out_min == *width) {
             *height = in_height * ((double) *width / (double) in_width);
         } else {
             *width = in_width * ((double) *height / (double) in_height);
         }
     }
 ...

case
input width > height:
2048x1024 => -w512 -h512 -a => 512x256 (OK)

input height > width:
2048x1024 => -w512 -h1024 
=> -w256 -h256 -a => 256x512 (should be 128x256)