When you update a blog post, Wordpress stores the old version of your post as a revision document. If you make very many updates, these revisions can have a significant impact on your database size. On some of my Wordpress sites, half the blog posts stored in the DB were revisions.
If you're sure that you don't need these older versions of your blog posts, you can delete them permanently with this MySQL command:
You might want to make a database backup first.
DELETE FROM wp_posts WHERE post_type = "revision";