Why do some SVG scale and others don't?

I've come across a few sample SVG images that, when viewed directly in a browser, appear to be the same size (say 100px) no matter how wide the browser is, and other SVG images that stretch to the entire width, maintaining their aspect ratio.

Looking at the source, I can find no clear differentiator between these two appearances. How do I modify a fixed-size SVG image to scale automatically?