RE: New Italian Q500
Nose length is set by balance requirements. Having a smaller surface area in the fuselage is a method of lowering its drag. More important, of course, is the shape. You want the air to stay laminar for as long as possible. You can do this with the shape (having the fuselage width increase as you move further back), or as in the case of a muffler the heat does the same thing to a constant section.
But eventually, you have to contract the fuselage. It's best done behind the wing, to avoid airflow separation at the wing root/fuselage junction. At that point, the air is most decidedly turbulent, so the method of reducing drag is to minimize the surface area. However, this contraction can not be too fast, or drag goes up again. You can pinch the tail boom of the fuselage in and achieve a pressure recovery area, which actually extracts some of the energy from the turbulent flow.
Listen carefully to airplanes in flight, the more noise the airframe makes, the more drag they are producing. Especially in the turns.