iOS 6.1 fixed a bug that has been a thorn in my side for a while, and I still haven't seen much about it on any other web site.
Are you family with the "Other" data in iTunes? It may creep up to several gigabytes in size.
It could be lots of things. Album art cache, image thumbnail cache, etc.
For me, it was SMS Attachments. Some time back, my friend started sending me "funny" pictures via text message. So I started sending funny images back to him, and then to other people. After iOS 6 came out, they went from just funny pictures to animated GIFs. Some of these were quite large, and would slow down the messaging app.
No big deal, right? I could just delete the image after sending it. Right?
Well, it turns out iOS didn't actually delete these images. It just removes them from the conversation. They were still on my device.
How did I find out? After my "Other" got so big that my free 5GB iCloud backup was FULL, I deleted everything from my device. All apps, all music, everything. Even with a "blank" device, my "Other" was not changed.
I then backed up, Restored my device, and restored from backup. "Other" was still not changed. My backup was several gigabytes in size.
So what did I do next? I checked out what was in my backup. You can use something like iBackupBot or iExplorer.
What did I find? Pictures I had sent (and LONG deleted) from a year ago. Pictures people sent me. Huge GIFs I had sent others. A huge (20MB) GIF I had sent to multiple people was saved multiple times.
My backup was nothing but "deleted" images.
/Library/SMS/Attachments had 256 folders (00 through FF), filled with hundreds of images and video, long "deleted" from Messages. Heck, I had even deleted all conversations out of Messages, but the files remained.
The only way to modify the contents of /Library is with a jailbroken device.
iOS 6.1 finally fixes this.
I tested it by sending a lot of pictures to people, then deleting them. I used two different devices for this test.
Device with iOS 6.1 (installed fresh): Tons of images sent, then deleted. Backup is clean.
Device with iOS 6.1 (upgraded from iOS 6.0): Tons of images sent. Only images deleted after iOS 6.1 was installed were removed. Images deleted before iOS 6.1 was installed still remain in the backup.
Some threads on this:
http://www.reddit.com/r/iphone/comments/17eec6/is_your_other_data_storage_quite_large_want_this/
http://www.reddit.com/r/iphone/comments/15nan4/sms_attachments_are_apparently_kept_forever_why/
http://thetecherra.com/2012/10/31/are-your-icloud-backups-suspiciously-large-heres-why/