The difference in volume between a 7 inch and 8 inch auger in 12 inches of ice is about 60 square inches of ice that you need to remove to finish drilling the hole (~341 square inches vs ~402 square inches). That might not be that much if you're drilling 1-3 holes per trip, but it definitely adds up if you're punching a ton of holes. Ten 8-inch holes is the same amount of material as twelve 7-inch holes. Over a day/weekend of drilling, it might wear you out faster.