What BigEd said, except for the water. At least in my system the boil off rate is almost the same between a 5 gal. batch and a 10 gal. batch. Because of that I need to calculate sparge water for each batch size.
Good point about the water. I was just talking about the grains & hops but assuming the kettle is the same size with the same surface area and the same burner you will want to account for a higher percentage of boil off with the smaller recipe.