This is largely due to the “Post Revisions”, which get auto-saved.
If you edit a post a few times, a huge list of revisions gets saved to the database. You can see these at the bottom of the post editing page.

This post has easy methods to disable and delete the post revisions, which are automatically cached by WordPress.

If you don’t use or need the post revisions which Worpdress has already saved for you, then you can delete them all by using the SQL query below:

You can also disable it from executing to start with by opening wp-config.php in your blog’s home directory.
Search for:

And add this under it:

There is also a great WordPress Plug-in called WP-CMS Post Control, which gives you more control over how posts are edited.
This great WordPress plugin will allow you to:

• Force standard browser upload instead of Flash upload
• Turn off the revisions feature
• Turn off the auto save feature
• Create a collapsable message panel that appears below the write panel
• Control which options are shown on the post page

I prefer to use the manual method, as too many plug-ins for WordPress can cause problems.

I don’t know anyone that has found the post revisions that WordPress saves, usefull.
Maybe in a future edition (v2.7 hopefully), they will include an option to disable from the admin control panel. But untill then thousands of blog owners will have to do it manually or use a plugin!