Featured image downsized

Hello,
When using FIFU on a site I’m putting together (localhost) the featured image is downsized to 1000px wide. Theme has width 1420.
Image source looks like this:

<img src="https://imgurl.jpg" class="attachment-post-thumbnail size-post-thumbnail wp-post-image" alt="post title" itemprop="image" data-large_image_width="1600" data-large_image_height="1067" width="1000">

Any idea where the width=”1000″ is coming from and what I can do to make it go away?

(When using WordPress’ built in featured image the width is 1420px, as it should be)

Regards, Th