Spend some time to get an accurate size.

def measureSize(self, diff, chunkSize):
        """ Spend some time to get an accurate size. """
        (toUUID, fromUUID) = self.toArg.diff(diff)
        isInteractive = sys.stderr.isatty()
        return self.toObj.diff(self._client.measureSize(
            toUUID,
            fromUUID,
            diff.size,
            chunkSize,
            isInteractive,
        ))