an inch deep and a mile wide

An inch deep and a mile wide is a cliche used for when something may seem extravagant and impressive at first, but is actually quite unimaginative. When talking about a person, this typically means that they know a lot about everything, or at least act like they know about a lot of things, but in reality dont know much about any given topic. An inch deep is the reality while a mile wide is the hype.