In Wordpress, when you save a post, it stores automatically a copy of the previous content of the post.
This can be a real time saver in case you want to restore an old version of the post, or if you accidentally deleted the content while editing it.
However all those previous versions of the post are stored in the database using a valuable space that can be used for other posts.
So if you don't want Wordpress to keep storing these copies of posts, add the following line to the wp_config.php file:
define('WP_POST_REVISIONS', false );