Grandiosity is a characteristic of being so ambitious or extravagant that you seem pretentious. The grandiosity of that new house down the street, with all of its turrets and columns, is a little ridiculous.
Someone's grandiosity might come across in the way they brag about their amazing achievements, blowing them up to be even bigger — or more grand — than they actually are. Or you might betray your grandiosity by making outlandish plans that don't seem quite realistic: "Later, I realized the grandiosity of my idea to start a cookie business that would make a million dollars in the first six months." The Latin root is grandis, "big."