The sun is _A_ bigger than the moon.
这是主系表结构,主语是the sun,系动词（即am is are)是is,表语是后边的bigger than the moon.和the sun is big.是一个道理,不过用了比较级.
如果再分细的话,than是个介词,the moon做介宾,much是副词做状语,修饰bigger