YOU SAID:

My tongue is big But not as big as his

INTO JAPANESE

私の舌は大きいですが、彼の舌は大きくない

BACK INTO ENGLISH

My tongue is big, but his tongue is not big

INTO JAPANESE

私の舌は大きいですが、彼の舌は大きくありません

BACK INTO ENGLISH

My tongue is big, but his tongue is not big