I am no automobile designer, but the BMW pics above I think illustrate how well the Panamera designers handle the rear proportions and slope into the rear bumper.  The BMW is awkward and out of proportion because its designers have remained committed to a squared off, traditional rear design.  That consequently requires big taillights, a ridiculous spoiler, a massive bumper, and a  "butt in the air" look.    With a more sloped, rounded off design like the Panamera,  the roofline from front to back flows better, the taillights are more proportional, and the rear of the car sits lower and more pleasingly on the road.