@staticmethod
def _sort_labels(label_data):
    """Returns the labels in `label_data` sorted in descending order
        according to the 'size' (total token count) of their referent
        corpora.

        :param label_data: labels (with their token counts) to sort
        :type: `dict`
        :rtype: `list`

        """
    labels = list(label_data)
    labels.sort(key=label_data.get, reverse=True)
    return labels
