Small version of image loading in

Hi, not sure what the scope of your support is (I'm new to WPUM!) and this may not be in your remit but...

On my site home page... http://www.lifeessentials.org.uk/ – the main background image to the hero is pretty lo-res. The original selected image is 2000px wide but when I check the console it's loading in a tiny version (http://www.lifeessentials.org.uk/wp-content/uploads/2017/02/night-sky-wide-col-2000-uai-516x267.jpg).

I do have WP Smush installed and I 'think' this wasn't doing this before that (though I was having some issues with WP Fastest Cache (now deactivated). Can you think of anything that would be causing my install to show a smaller version than I want? Is it a WP Smush setting or something else?!

Thanks!