The apple is bigger than previously thought.

The apple is bigger than it was 1 month ago.

这二个句子都是正确的。

第一句可以还原was, 得到：

The apple is bigger than was previously thought.

第二句还可以省略it was,得到：

The apple is bigger than 1 month ago.

但不能只省略it 而保留was, 下面是错句：

The apple is bigger than was 1 month ago. (错误）

由此可知以上二个比较分句是不同的结构。

如果比较分句的主语及谓语动词与主句的主语和谓语动词相同，比较分句可以省略主语和谓语动词，即二者同时省略，不可以只省略主语而保留谓语动词。

只有二种情况下，比较分句可以省略主语，而保留谓语动词：

1 比较分句的主语为比较短语的对应项。这时，比较分句的主语是必须省略的。例如：

More people came than had been invited.

2 比较分句的主语为主语从句（形式主语it，及外置的真正主语从句）。此时，要么不省略，要么形式主语it 连同that从句一起省略。不可只省略that从句而保留形式主语it。例如：

The apple is bigger than  it was previously thought that it was.

这个句子中第一个it为形式主语，第二个it为人称代词，指代the apple. that从句为比较分句中的外置主语从句。此时，可以省略形式主语it ，同时省略that 从句，得到：

The apple is bigger than was previously thought.

was可以进一步省略，得到：

The apple is bigger than previously thought.

如果保留形式主语it则为语法错误：

The apple is bigger than it was previously thought.