Wow, the S197 is actually smaller?!It's not smaller.
S197: 187.6" Length, 107.1" Wheelbase
S550: 188.3" Length, 107.1" Wheelbase.
Autoblog is reporting the same thing, a weight gain: http://www.autoblog.com/2014/06/01/2015-ford-mustang-weight-gain-300-pounds/
Looks like the new Mustang will weigh about the same as the Camaro, and inch closer to the fat ass Challenger.