Do you also have backups from your other iOS devices listed in your sig?
I have also found that sometimes multiple backups are saved for a device instead of only the most recent backup. I called Apple and they said I could delete older backups, if desired, in order to not waste the disk space. You can check the status of iOS devices backups and delete them by launching iTunes and checking Preferences...>Devices.
Normally each time you back up your iPhone it is an incremental backup of the previous one, but there are sometimes good reasons why you might want to "archive" a backup, done by right clicking the backup in iTunes prefs. This means that back up is preserved (has a date next to it), and that the next backup is a whole new one instead of incremental.
If you have more than one backup on the same device it may be an archived one, which if you don't need any more you can delete in iTunes.
Because the backups are incremental, the size will grow as you do more, so you could delete the existing backup and do a fresh one.
Your backup may be large because you have a lot of your own material on it as my previous reply in #9.