Do you mean the tongue just looks very long, or does it actually swell up and get really big and fat??
Dogs have no sweat glands so they release heat from their body through panting with their tongues hanging way out.. that is 100% normal. If the tongue swells up and gets big and fat, though, that's not normal.
Dogs cool themselves through their tongue so it gets longer and wider to allow more surface area to reach the air. It doesn't actually grow, it just spreads itself out thinner which makes it appear larger. There is a thread somewhere on this forum showing dogs with really long tongues.....it was funny to look at all of the pics.