def METHOD_NAME(font_filename1, font_filename2, args):
  """Prints a size comparison for two fonts.
  If so flagged (--diff_tables), prints per-table size change.
  Args:
    font_filename1: The first font to compare.
    font_filename2: The second font to compare.
  Returns:
    String describing size differences.
  Raises:
    OSError: If either argument doesn't point to a file. errno.ENOENT.
  """
  if not (os.path.isfile(font_filename1) and os.path.isfile(font_filename2)):
    raise OSError(errno.ENOENT, 'Missing at least one of %s and %s' % (
        os.path.basename(font_filename1), os.path.basename(font_filename2)))
  font_sz1 = os.stat(font_filename1).st_size
  font_sz2 = os.stat(font_filename2).st_size
  result = '%s (%d) vs %s (%d) (%+d)\n' % (
      os.path.basename(font_filename1), font_sz1,
      os.path.basename(font_filename2), font_sz2, font_sz2 - font_sz1)
  if args.diff_tables:
    result += DiffTables(font_filename1, font_filename2)
  return result