In an interview with DC.watch (google translation here) the Sony A7-A7r developers explained that a Sony A7 with NEX-7 alike design would have become considerably larger. And the EVF placement in the middle is an advantage if you sue larger A-mount lenses (you eye is on the optical axis).
There is also a hint about the FE lens roadmap: They confirm that faster Sony FE lenses are in development (faster than the Zeiss FE 55mm f/1.8!). Great news!