改错。When comparing with zhe size of the whole earth,the biggest ocean doesn't seem big at all.

comparing --- compared

注释：
compared with …与……比较起来

compared,后面的主语是海洋,而海洋与地球比较起来,在这个比较中海洋是被动的,因为海洋不会主动和地球比大小.
When (it is)compared with the size of the whole earth