Background-size with SVG squished in IE9-10

Be sure that your SVG has a width and height specified. If you're generating it from Illustrator, ensure that the "Responsive" box is unchecked as this option removes width and height.

Adding a width and height to the SVG as mbxtr said nearly worked for me. I needed to add preserveAspectRatio="none slice" as well to get it working responsively in IE.