WordPress has many default image sizes. When you upload an image on your website, it creates all the image sizes for your new upload and take more space on the server. Few of the default image sizes are unnecessary and they may also confuse your media configurations. It’s better to remove these image sizes permanently.
Instructions: Add the following code to the functions.php file of your WordPress theme.
update_option( 'thumbnail_size_h', 0 ); update_option( 'thumbnail_size_w', 0 ); update_option( 'medium_size_h', 0 ); update_option( 'medium_size_w', 0 ); update_option( 'large_size_h', 0 ); update_option( 'large_size_w', 0 );
You may also enjoy removing featured image meta box in WordPress.