To decrease your wordpress database size follow the below steps.
In wordpress when you editing a post, it automatically saves at a specific time interval. The default time is 60 seconds.To save your database just increase the time delay.To do this, Open your wp-config.php file and add the below code.
define('AUTOSAVE_INTERVAL', 500 ); // seconds
Post revisions is one of the great feature in wordpress but it also increase your database size.It allows reverting to a previous version of that post or page in wordpress.
To disable this function,Open your wp-config.php file and add the below code.
define('WP_POST_REVISIONS', false );
Or just limit the Number of Post Revisions replace false with number.