A quick gander of the WordPress forums will easily show that there are many people who wish they had a backup of their database but did not.
Another problem, however, is the size of the database backups. Making sure to have a few separately-dated backups to revert to in case of a problem is recommended, but sometimes the databases are so large, that is not easy to do.
Many plugins store a bunch of data in your database and increase the size of your database. Plugins which gather and collect data, statistics, external information, etc., or plugins that block spam can be especially problematic in adding a lot of unnecessary data to your database.
For this reason, among many others, be very careful of plugins. Try to avoid them whenever possible and, if you must get by with a plugin, be sure to pick ones that are as small as possible and are proven compatible and affective with the latest versions of WordPress.
If you have problems with database size, it may be best to backup your database manually. In PHPMyAdmin, you have the option to select which tables will, or will not be backed up. Ignore the extra plugin tables which are not needed. Only include those tables that are important to your blog’s data in your backup file.
Generally image files are not stored within the database, but if they are, it would be best to remove them. Extra content like this within the database can add a great deal of unnecessary space to your database size.
Be sure to repair and optimize your database too. It will help get rid of overhead and speed up your database. See: http://helpdesk.bluehost.com/index.php/kb/article/000496