Grandiosity can be a symptom of both mania and hypomania, but delusions of grandeur are more severe and only occur in bipolar I disorder. The difference between them can be a fine line.
Ever feel like you can do something better than anyone else? That others are inferior to you because of your wonderfulness? Boast about abilities you don't have - at least, not to the degree you think you have?
Those are examples of grandiosity. Whether they become delusions of grandeur depends on the...
