Fluida is designed with wide featured images in category lists. You can adjust the featured image height from the theme’s options (under Post Information > Featured Image in the customizer), but the width will always be that of the post width (which on the posts in one column layout is equal to the site general width).
If the images your posts display were uploaded before you’ve activated Fluida on your site, you should regenerate thumbnails for WordPress to create better proportioned (wider) image crops for the theme.