docs: def diff_sizes(a, b, progressbar=None):
      """Return list of tuples where sizes differ.
  
      Tuple structure:
      (identifier, size in a, size in b)
  
      Assumes list of identifiers in a and b are identical.
  
      :param a: first :class:`dtoolcore.DataSet`
      :param b: second :class:`dtoolcore.DataSet`
      :returns: list of tuples for all items with different sizes
      """
      difference = []
  
      for i in a.identifiers:
          a_size = a.item_properties(i)["size_in_bytes"]
          b_size = b.item_properties(i)["size_in_bytes"]
          if a_size != b_size:
              difference.append((i, a_size, b_size))
          if progressbar:
              progressbar.update(1)
  
      return difference