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 depth of your belief and, often, what you do about it.
There are so many ways grandiosity can be expressed that it's impossible to list them all, but here are a few:
Alice is a very good commercial artist, but during mania she believes she has more talent than she does. Every piece she produces is better than anyone else's. She becomes condescending to the other artists in her office and goes around puffed-up with an exaggerated sense of her own importance.
However, if Alice starts buying expensive art supplies, painting canvases deep into the night, and telling everyone she's been contacted by the Metropolitan Museum of Art and the Louvre about hanging her paintings, maybe even quits her job to concentrate on painting full time, she's slipped into delusions of grandeur.
Both grandiosity and delusions of grandeur often have a religious component, as demonstrated here:
Kevin has always been an faithful churchgoer, but as his mood gets elevated, his religion takes on a new meaning. He boasts to others about his deep knowledge of the Bible and becomes lofty and sanctimonious with his co-workers. His grandiosity is hard at work.
But if Kevin believes God is speaking directly to him, causing him to start shouting on street corners telling people to repent and be saved, or starts believing he is next incarnation of Jesus, he has delusions of grandeur.
Grandiosity can take forms that aren't as obvious, too. When garden catalogues being arriving in the mail in winter, Martha believes she can redo her entire landscaping in the spring. She orders hundreds of plants (at no small expense) and spends hours planning where she'll put them because she has extravagent and unrealistic ideas about her capabilities.
Deep down she may know she's going overboard, but
it doesn't stop her - until the plants arrive in spring, and she gazes over them with a sort of horror. Her grandiosity has faded in the intervening months, and she knows full well there is no way she can possibly remove all her existing gardens and get all the new plants in the ground.
She never told anyone of her great plans, never flaunted her super-gardener abilities. She wasn't delusional - simply grandiose... and now she will pay for that episode in the hardship of her reckless spending and the guilt of watching plants die.