A
ghost in a dream can represent a part of yourself that you don't understand or you don't like.
The fact that the ghost was in the bathroom makes me think that the ghost is part of yourself that you aren't happy with - a bathroom is where you get rid of waste.
Since the food you eat is what goes into making your body,and therefore makes what you are (You are what you eat), your mother saying that it's easier to redesign the bathroom than redesign your diet could mean that it's easier for you to try to get rid of the part of yourself that you don't like than try to accept it and recognize that it is part of who you are.
Since you see a short, fat ghost and your dad says he saw a very tall ghost, this dream could also have something to do with your body image.