Adults must be careful about what they say within the hearing of children. The saying refers to the large handles (ears) sometimes attached to small vessels.
little pitchers have big ears
Young children often overhear something they should not. For example, Don't use any swear words around Brianlittle pitchers have big ears. This metaphoric expression, which likens the curved handle of a pitcher to the human ear, was already in John Heywood's proverb collection of 1546.