I have a Time Capsule 802.11n 1 TB at work with some free space on it, and I decided to make a second backup on it, using Time Machine. I have about 358 GB used on the hard drive, and 25 GB of that are in a folder that I excluded from the Time Machine Backup, so the actual amount of data to be backed up is about 333 GB. Yet my initial backup (before any incrementals) took up 477 GB of space on the Time Capsule. Even after running
hdiutil compact on the sparse bundle, I was only able to reduce its size to 455 GB. Isn't that still excessive? I understand there may be some overhead, but really, isn't ~40% overhead excessive?
A piece of information that may be relevant to the question: Unfortunately my rMBP has no Ethernet and I haven't purchased the USB-to-Ethernet adapter either, so the backup proceeded very slowly under 802.11n. Unable to finish the backup in a single day, I took the rMBP home each night, and on the following day, I would let Time Machine resume the backup (it did seem to be resuming since the amount to be backed up decreased every day). Eventually, a week after starting the backup, I finished it for the first time, but with a size of 477 GB as mentioned above.
Edit: here's the output of
sudo tmutil listbackups | tr '\n' '\0' | xargs -0 sudo tmutil uniquesize, showing backup dates and their
16.6G /Volumes/Time Machine Backups/Backups.backupdb/MacBook Pro/2014-01-15-182027
3.4G /Volumes/Time Machine Backups/Backups.backupdb/MacBook Pro/2014-01-22-161530
623.1M /Volumes/Time Machine Backups/Backups.backupdb/MacBook Pro/2014-01-23-160536
1.0G /Volumes/Time Machine Backups/Backups.backupdb/MacBook Pro/2014-01-23-171738
1.1G /Volumes/Time Machine Backups/Backups.backupdb/MacBook Pro/2014-01-23-182535
4.7G /Volumes/Time Machine Backups/Backups.backupdb/MacBook Pro/2014-01-24-144806
This is, of course, a few days after the question was asked, so there are new backups. But notice that there is only one backup made before the question was asked.