Not sure what you mean... A Prius is actually one of the single best production cars around in terms of aerodynamics (Cd around 0.24).

That stubby tail is actually done on purpose because it reduces drag, not increases it as you claim. A lot of aerodynamics are pretty counterintuitive. Look at the big table of drag coefficients here: https://en.m.wikipedia.org/wiki/Automobile_drag_coefficient

For the Bugatti, they probably weren't going for an ultimate low drag coefficients. They need to make sure their giant engine is getting enough air, which leads to lower performing shapes. Related, they also will want a lot of downforce to make sure the car stays on the road at high speeds; to do this they build a relatively non-aerodynamic car.