With WordPress 2.6, post version tracking mechanism was introduced. For example, every time you “Save” a post, a revision is written to the database. If you do not need this feature you can easily turn it off by adding one line to your wp-config.php file, found in the installation directory of your WordPress site:
If you have run a blog with revisions turned on for a while, chance is you will have a lot of revision posts in your database. if you wish to remove them for good, simply run this query (for example using the mentioned WP-DBManager) plugin.
DELETE FROM wp_posts WHERE post_type = "revision";
This will remove all “revision” posts from your database, making it smaller in the process.
NOTE: Do this with care. If you are not sure what you are doing, make sure to at least create a backup of the database first or even better, ask a professional to help you.