So I downloaded the 977M 2015-01-31-raspbian.zip, dd-ed it to the card, expanded the fs, installed ~100MB of software did a backup and was left with a 784M raspbmc_backup_20150208_clean.img.7z
now LZMA2 compression aside I'm guessing the official images do not have
free space zeroed like the upstream raspbian. Hope there is nothing sensitive in there
