iOS was storing 2, 3, sometimes even 4 copies of the same voice memos.
var/mobile/Media/Recordings was over 2GB in size. I know I only have about 90 voice memos (and only 30 in the Voice Memo app), so this was strange. Going into the folder revealed 357 voice memos, most of them stored in at least triplicate. (Side note: I'm not even sure why iTunes backs up voice memos that have already been copied to iTunes; apparently it just backs up that whole folder). So I backed up the voice memos that I had already synced with iTunes, unchecked the box that said "Include Voice Memos" under Music sync settings, backed up my phone, and then destroyed that whole folder. I then turned "Include Voice Memos" back on and synced my phone and all 90 of them showed back up in that folder, while only 4 showed up in the Voice Memos app. I thought that was odd, so I synced again. Now there were still only 4 in the app, but 170-something in that folder. iTunes was copying the same ones again, and since they weren't referenced in the Recordings.db file, it apparently didn't know they were already there. It appears that if you have any orphaned voice memos (and I'm not sure how 86 of mine now are), they will be copied again and again to your phone. I just turned off syncing voice memos altogether and deleted all the files in that folder again. After removing those, my iCloud backup was down to 2GB.
Next, I found that:
SMS/Attachments was 1GB. Under /var/mobile/Library/SMS, the SMS.db file itself was only 42MB. This is for entire threads going back to 2009. I'm guessing they compress the hell out of this. But the Attachments folder was 1GB. That seems plausible to me, since I send pics and video a decent amount, but I read about an issue where iOS was saving SMS attachments even after you had deleted them. This issue has been fixed in 6.1, but I have to wonder if there are any orphaned pictures/videos in there. I'm tempted to back up the entire database and then wipe the folder. Doing so would bring my backup down to 1GB in size. Which seems almost.. normal.