建议你看这个解答前，先看我对你的上一个问题的解答。

The box is much smaller than is useful.

此句正确。这里我们可以理解为than what is useful to a reader. 这里的比较对象可以理解为box, what实际上=the box that 所以也可以理解为：

The box is much smaller than the box that is useful. 如果理解为than it is useful that the box should be small.在一定语境也是可以的，语境为我需要小盒子，但这个盒子小于我需要的大小了，言外之意，盒子太小了，无法用。

请看几个外刊例句：

1. Break content down into chunks that are smaller than is useful to a reader. （道理同上）

2. A consequence of this phenomenon is that the profile user might be tempted to use profiles that are more complex than is useful or necessary, simply because the profiles appear to be much better than they really will be. （从这个例句的useful or necessary可以看出，两个用法是一致的）

3. But there is frequently more inflammation than is useful. （相当于than what is useful或the inflammation that is useful)

The box is much smaller than is useful.

根据剑桥英语语法，than从句省略的主语既不是比较对应项，也不是形式主语it+外置主语，这种情况下省略主语和保留谓语动词即为语法错误。所以这是一个很明显的错误句子。你所谓的补全是错误的，因为useful的主语并不是主语从句，而是the box。从语法结构上讲，改正这个句子很简单，把不该省略的主语it（指代box）加上即可。The box is much smaller than it is useful.

than 和 as 在从句中可以替代主语（很像关系代词）。

He worries more than is necessary. 他担心过度。

（不能说：... more than it / what is necessary.）

以上内容摘自《牛津英语用法指南》第四版256.1节

所以，下面的句子是完全正确的，没有必要也不可能补写：

The box is much smaller than is useful.