According to the folks who sell dried shiitakes, it takes a full pound of mushrooms to yield just 3 ounces of dried. But just as sun-dried tomatoes are much more intensely flavored than fresh, dried mushrooms yield much stronger flavor.
Dried shiitakes are hydrated, one way or another, before you eat them. They may be hydrated in a slow-simmering soup, or soaked in water before they go into the dish. Even when hydrated, though, dried shiitakes will be chewie–or, some would say meatier–than fresh.
This Brussels Sprouts and Shiitake Lo Meinrecipe is a great example of how to rehydrate dried shiitakes.
For more information on cooking with mushrooms, see 7 Ways with Mushrooms.