If you go to Settings then choose iCloud, scroll to the bottom then press Storage & Backup.
You then need to press Manage Storage. This will list your backups, and their total size. If you only have one iPhone listed, choose that and it will give you a list of all the apps that are backed up to iCloud. If you use the camera it's likely that a large percentage of your total backup will be taken up by the camera roll. You have the option of choosing which apps are backed up - so you could choose not to backup free apps.