I recently had a customer that had ~70,000 revisions on posts on his WordPress site. This caused his database to skyrocket to 400mb+. I discovered the cause of these revisions to be the plugin FeedWordpress. FeedWordPress was triggering WordPress to create massive amounts of revisions. Because of this the plugin I wrote, BackupBuddy, could not back up his site without running into PHP timeout issues. In this post is the solution I used to resolve this problem.
In the root directory of your WordPress installation, open wp-config.php in a text editor and insert the following line on a new line somewhere between the top ( looks like ) lines of the file:
define('WP_POST_REVISIONS', false);
Running the following SQL (change wp_ to your prefix) in phpmyadmin will clear out all revisions:
DELETE a,b,c
FROM wp_posts a
LEFT JOIN wp_term_relationships b ON (a.ID = b.object_id)
LEFT JOIN wp_postmeta c ON (a.ID = c.post_id)
WHERE a.post_type = 'revision';
Finally, run the following SQL query to optimize (clean up) your posts table:
OPTIMIZE TABLE wp_posts;