responsive images srcset not working

I've been trying to implement the new srcset approach to responsive images, using the following HTML

        <img class="swapImages"
        srcset="assets/images/content/large.jpg 1200w,
                assets/images/content/medium.jpg 800w,
                assets/images/content/small.jpg 400w"
        sizes="100vw"
        src="assets/images/content/small.jpg"
        alt="responsive image">

Im using chrome 40 and all I get is the largest image, resizing my browser, clearing the cache does nothing.

I read somewhere I had to polyfill, so I used the picturefill plugin, although chrome should support it.....still doesn't work.

I put together a demo page for it: http://ift.tt/1JwQKwD

What am I doing wrong/not getting?

Any help is massively appreciated.