the_post_thumbnail displaying wrong size images

Hi there, our website http://www.yorkmix.com uses a custom-built child theme with twentyseventeen as the parent theme. All works fine but the standard loop we are using within archive.php displays the wrong size image. In fact, it appears to display different image sizes for different posts. You can see it here - https://www.yorkmix.com/category/life/

We have as similar issue on the homepage - https://www.yorkmix.com

What are we missing?

YorkMix