Going from S197 into S550, focus groups didn't want a smaller Mustang. People, in general, didn't want a smaller Mustang, in fact, they wanted more room inside.
GM didn't listen to people, shrank Camaro, and look at them now...
The best we can hope for is a S650 that is not gaining any weight.
Another one is Challenger, its bones are heavy and it needs a new platform bad.
But, the moment FCA makes it smaller inside is the moment they start loosing customers.
Same with Charger. Some people like 'em big and roomy.