这句话从语法角度怎么理解？

An acre of average farmland produces only about 400 pounds of grain amarnanth, as against 2400 pounds per acre, or six times as much, for wheat.

上句中的as against... as much怎么理解呢？一般见到的都是as much as的结构啊。如果把上句作为一个语法题来考的话，我肯定要做错的。

请大家帮助分析下。鞠躬！

爱莫能助