docs: function diffSize (pathA, pathB) {
    return readSize(pathA)
      .then(sizeA => readSize(pathB).then(sizeB => ({sizeA, sizeB})))
      .then(({sizeA, sizeB}) => {
        return {
          sizeA: sizeA,
          sizeB: sizeB,
          diff: sizeA.diff(sizeB),
          sign: sizeA > sizeB ? '+' : '-'
        }
      });
  }