She is not fat! She's big boned...
Or do you mean she is "fluffy" (I have no idea where that got started, but it is pretty ridiculous)
The 55 aspect ratio gives the bike the proper tire height for accurate speedometer readings along with a sharper turn-in response, and it holds a curve like on rails which is very useful in the sweepers. The 50 series rear always wants to stand up straight in curves and need constant corrections which to me is very unnerving. With a bike as heavy as the C-14 the 55 rear makes "dancing with the fat lady" easier IMHO.