Images that do not have a width and height attribute are currently rendered to
256x256 pixel. This breaks the aspect ratio for all non-square images, and is
also quite small.
SVG does not require the width and height attributes to be present, see
http://www.w3.org/TR/SVG/coords.html#ViewportSpace. If not present, the size
to use is to be derived from the document including the SVG. In the case of
MediaWiki, this would be the thumbnails size, resp. the size given in the user
preferences for the size of images on image description pages. Also, the aspect
ratio defined by a viewBox attribute should be honoured, if present.
A patch that addresses there issues will follow in a minute.