I'm sending this to the list rather than as a support request in the hope that other people have noticed this and can add a me too which may help Andy track down the issue. I first noticed this yesterday and confirmed that it is still the same today, when I go to the backups page in the panel apparently my backups are taking no space, however if I mount the backups on my VPS all appears well. So barring Andy fininding or creating a really cool compression algorithm (and not having shared it) it appears there's an issue with the backups page in the panel.

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Robert Gauld
http://www.robertgauld.co.uk