In making the comparison, Angus uses the word "now," which is significant. It implies that, at this stage of his rule, Macbeth feels the heaviness of the duty that he illegally claimed for himself. Angus suggests that Macbeth had took on too great a responsibility and was not fit for the task. He did not realize when he usurped the throne that it would put so many demands on him.
At this point in the play, the English troops under the leadership of Malcolm, Siward, and Macduff are gathering to launch an assault on Macbeth's castle and overthrow him. Caithness has reported that Macbeth has fortified his castle at Dunsinane but that he cannot rely on the support of those he rules, for most have turned against him and others believe he is mad. There are very few who still support Macbeth and believe in his courage. It is evident that the odds are stacked against him at this juncture.
The comparison of the title to a cloak emphasizes the expression that 'clothes do not make the man,' since Macbeth does not, and has not, displayed the qualities of a king. The metaphoric kingly robes that he has attired himself with are ill-fitting. They are much too large since he lacks the greatness, character, or ability to fill them.
Angus mocks him and compares him to a "dwarfish thief." This comparison is effective, as dwarves were commonly ridiculed and seen as objects for others' fun and pleasure. Angus is suggesting that Macbeth has made a fool of himself by trying to be king when he stole (usurped) the throne. The rogue tyrant was obviously not fit for the position and he is, at this time, feeling the pressure and will evidently dearly pay for his faux pas.
In Act l, Scene 3, clothing imagery is also used. Ross tells Macbeth that he has been awarded the title Thane of Cawdor. Macbeth asks why Ross dresses him in "borrowed robes" since Cawdor was still alive. The robes symbolize the title of thane. Ross then explained that Cawdor was to be executed for his betrayal and his title would be bestowed on Macbeth.
On a previous occasion, in Act l, scene lll, clothing imagery is also used. Ross tells Macbeth that he has been awarded the title, Thane of Cawdor. The general ask why he dresses him in 'borrowed robes' since Cawdor was still alive. The robes symbolize the title of thane. Ross then explained that Cawdor was to be executed for his betrayal and his title would be bestowed on Macbeth.
It is ironic that Macbeth should ask this portentous question, since it alludes to him later, not only 'borrowing' king Duncan's robes, but actually stealing them.
In Act 5 Scene 2, Angus tells the other men that Macbeth's feels "his title hang loose about him, like a giant's robe." At this point in the play, the men suspect that Macbeth has been complicit in the murders that have lately occurred, and they question whether Macbeth is fit to remain king. The title of "king" is a large honor to bear, and Angus compares the role to a "giant's robe." Angus then goes on to compare Macbeth to a "dwarfish thief," meaning that Macbeth stole the title of king as opposed to rightly assuming it. Further, Macbeth's dishonorable character is "dwarfish" and small, not at all the "large," loved man whom he should be as king.
Earlier in the play, Macbeth makes another reference to "robes." In Act 1 Scene 3, Macbeth asks Angus why he dresses him in "borrowed robes" upon hearing that he is now the Thane of Cawdor. To Macbeth's knowledge, the actual Thane of Cawdor lives, but he does not know that Duncan has taken the man's title away as a punishment. Macbeth feels like he is being given an honor that is not really his, hence the term "borrowed."
The references to clothing are metaphors for characters adopting personas and roles throughout the play, just as Macbeth is "trying on" the position of king.