The answer to the question above would be that the metaphor "this house is a closet" refers to the size of the house. The house seems to be so small that the speaker compares it to a closet.
A metaphor is a figure of speech that describes an object or action in a way that is not literally true, but helps make an idea or comparison. In addition to this, it generally states that one thing is another thing. In this case, the speaker is comparing the house to a closet because he thinks it is reallly small.
