Post thumbnail large not showing the correct size

I’m trying to output the lagre version of the post thumbnail, using:

<?php if( has_post_thumbnail() ) the_post_thumbnail('large'); ?>

The image that is shown is set by WordPress to 640x360px when it should be 1140x641px;

The original image is 2000x1125px, which should scale down exactly to 1140×641, and the container that the image is inside is roughly 1140px wide.

There is no CSS code affecting the image other than max-width:100%, which I turned off to see if this was the cause. It wasn’t.

Any suggestions as to what the issue might be?