What is WordPress Revisions
Revisions mean WordPress will save a record at each time you Save/Update, then you can track the changes you made. You can get more details at WordPress.org Revisions support page.
Why disable WordPress Revisions
For reducing the database size of course, and by this you can also speed up your site.
I assume you also understand the disadvantages, right?
How to disable WordPress Revisions
Open wp-config.php located in your WordPress root directory and add the following code:
define(‘WP_POST_REVISIONS’, false );
Notice: This code will disable the future revisions but not delete your past revisions
Delete Old Revisions Data
To delete all previous revisions, you will need to run the following SQL query on your WordPress database
DELETE FROM wp_posts WHERE post_type = “revision”;