I did a little "digging" on it (really, I should have known). When the profile of a tyre is given, it's in percent of the width of the tyre - as the rear wheels are wider, the profile becomes lower, relative to the width of the tyre.

So, Mark is right, you'd have to go for a rear rim larger than that in the front to even out the visual difference the design provides, although - I'm not sure such a combination would be recommended on the E60 or any given luxury sports sedan?

-paasan

 

I saw a Chrysler Crossfire recently with original equipment 19" front and 20" rear. I'm sure some other models feature larger rears as well to balance out the optical illusion created by certain designs.
Mark-Yorkshire

 

The Corvette always did that as well.