Spec includes SVG http://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html , but xdg-icon-resource does not support them
Attempting to install an SVG icon gives:
xdg-icon-resource: icon file to install must be a *.png or *.xpm file
Try 'xdg-icon-resource --help' for more information.
This is perhaps a duplicate of bug #7837 , and it was reported in Ubuntu's Launchpad https://bugs.launchpad.net/ubuntu/+source/xdg-utils/+bug/790449
Also, for scalable icons like SVG the --size argument should be optional (or not allowed at all)