If you have a lot of image sizes enabled, WordPress will create many different versions of every image you upload. If you have 12 image sizes, then WordPress will create at least 12 variations of every image you upload. Removing/Disabling unused image sizes can save you a lot of server space and bandwidth.
To remove an image size, copy and paste this snippet into your theme’s functions.php file:
The remove_image_size function removes image sizes from WordPress. It takes a string argument which is the name of the image size.
BUT that function doesn’t work on WordPress’ default image sizes.
If you want to remove a default image size that is built into WordPress, then you need to use this snippet:
update_option( 'thumbnail_size_h', 0 );
This is a workaround that lets you remove any image size, even if it’s a WordPress default image size.