Folders To Ignore
There are many folders/files that could be drastically increasing the size of your Wordpress backups. These include cache files created by various plugins.
The following information is based of my personal experiences only! (i.e. there will be other Wordpress plugins that create backup trouble, but I don't use them).
Add Watermark is a useful plugin for dynamically adding watermarks to images that are served to users. But is also can create a large cache directory, which can clutter backup operations.
You can safely ignore any files in the folder wp-content/plugins/add-watermark/cache/.
NextGen can create a rather large number of cache/backup files in wp-content/gallery/.
You should ignore all files in the wp-content/gallery/cache/ folder.
You should also ignore all files that reside with any subfolder named dynamic of wp-content/gallery/ (e.g. anything that matches the glob syntax wp-content/gallery//dynamic/).