Thumbnails are too big

@dmitrynel Are the thumbnails you’re referring to appearing different between your AMP and non AMP URLs? If so please share your site URL and an example so we can inspect your setup.

@dmitrynel Thanks for the update, I can see you’re using AMP in reader mode. You’ll find information below on how to modify the reader mode templates.
https://amp-wp.org/documentation/how-the-plugin-works/classic-templates/

The custom CSS section section in particular may be useful in your case. You can apply CSS changes to the featured image by applying changes to the amp-wp-article-featured-image class. The below is an example:

add_action( 'amp_post_template_css', 'xyz_amp_my_additional_css_styles' );
function xyz_amp_my_additional_css_styles( $amp_template ) {
	?>
	.amp-wp-article-featured-image {
          width: 50%;
	}
	<?php
}