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.

What does Angus mean in Macbeth?

Angus. Meaning: Now Macbeth feels his royal title is too big for him, like a giant's robe on a midget. Theme: Clothing Motif. Where are these Thanes heading to? They are heading to Birnam Wood.

What does the reference to the clothing in lines 21 22 mean Macbeth?

It means that Macbeth is in stolen robes. A giant's robes wouldn't fit a dwarf and the use of the word thief makes the reader think about stolen goods.