The Yamaha RX-A1070’s Dolby Center Spread is only functional when playing two channel signals using Dolby Surround with Center Spread ON. It should spread vocals out evenly to the front channel speakers when working properly.
The loss of bass using the center channel speaker set to SMALL is presumably the result of a crossover setting for the center channel that is too low. YPAO will most likely have set the crossover settings of each speaker at or close to its basement.This will result in the subs not doing what they could and should be doing for you regarding two channel signals.
ALL of the speakers should be set to SMALL with the Extra Bass setting OFF. Set the crossover settings anywhere from 120Hz to 80Hz. Experiment with it and you will find your own sweet spot for each of them. Have fun with it and prepare yourself for the PB 2000 recommendations.