I'm trying to get the lazysizes.js plugin working (https://github.com/aFarkas/lazysizes)

On my images I have:

<img data-srcset="medium.jpg 768w, small.jpg 634w" data-src="small.jpg 634w" data-size="auto" class="lazyload">

However, to matter what size screen I view it on it always seems to use the medium.jpg image size.

I'm really puzzled how to set Lazyload up.

Would anyone know what I'm missing?