It’s the coupe’s rear/trunk panel!! If it was a bit wider like the S63 I think it’d be just right. Past the wider rear track, for me it just scrunches in too much. Just personal preference - I guess I’m a big car booty kind of guy 991 GT3 rear anyone?
I have always felt that the C63S Coupe looks odd in certain angles from the rear/side leading to a way too wide hips look compared to the very top of the car IMO. If the top half of the car was a bit wider like the E or S, the bottom width makes sense otherwise it doesn't feel well proportionate when viewed directly from the rear (for example, the angle shown below). When I first saw a C63S coupe on a highway in stop-and-go traffic I was directly behind, tailing the car, and this was way too obvious from that angle.
Also no two-tone option for the color with the Coupe due to lack of Pano roof option. I guess you could paint or wrap the car, but I wish MB just offered this option.
I feel the C63s sedan however has overall well proportionate looks from almost all angles (there are still few bad angles, but I have felt it's less compared to the Coupe in general). And yes, you get true two tone look by going with the Pano roof option. However I do wish they went with a slightly better tail-light design on the Sedan, may be with a little more width and possibly spanning across the trunk as opposed to the bare-bones simple look.
Again all of these are personal preferences and choices. Both of these pics are not of my cars BTW, just random ones I pulled off the internet to illustrate this. I haven't taken my car much outside other than to give a deep and thorough 3-hour car wash last week and few 15-20 min drives on surface roads and few highways to ensure I started and drove the car at least once a week.
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.