In Act 5, Scene 2 Angus says to the other men: "Now does he feel his title/ Hang loose about him, like a giant's robe/ Upon a dwarfish thief."...

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.