On my VPS I see hourly snapshots consuming 2.5GB -
what does this mean exactly?
That's how much of *your* files are available in the backup; this is
more like a 'virtual' size since the actual size is made up of a
series of (perhaps) small deltas applied to your initial backup.
Since Andy/rsync only cares about the sizes of the initial backup +
all the deltas to date, this isn't how much space is taken up in your
backup allowance.
Cheers,
L