The output of the paper mill is now three times as high as i

The output of the paper mill is now three times as high as it was in 1966.
此句在in 1966前面用了it was.为什么呢?
the irrigated area in this place is four times bigger than in 1978.
为什么这句中in 1978前面没有it was
两句话加不加it was的区别在哪?

as it was in 1966 是一个比较状语从句.it 指the output.这里的it was 可以省略.
the irrigated area in this place is four times bigger than in 1978.这里than后面省略了it was（即省略了主语和系动词）.
省略不省略it was意义没有舒适区别,不省略显得更正规,省略则显得更口语化.又如：
She is as beautiful as （she was） five years ago. 她跟五年前一样美丽.
Whe is as beautiful as (she was ) in 2003. 她跟2003年时的样子一样美丽.