How to reduce unnecessary thumbnail creation?

You can use a plugin for this.

If you want to do it by writing your own code, you may use remove_image_size().

add_action('init', 'wpse325870_remove_plugin_image_sizes');
function wpse325870_remove_plugin_image_sizes() {
    remove_image_size('small-A');
}

Note that you cannot remove reserved image sizes using this function.