Not a tuning and fueling issue at all HOWEVER the efficiency of an engine is dependent on exhaust gas velocity and the "pulses" in the exhaust system. These vacuum pulses created by each cylinder'* exhaust are what help scavenge the exhaust of the next firing cylinder and so forth and so on. The pulses are more important on the bottom end and if the exhaust pipe is too large, the pulses are weakened to the point they no longer help pull ALL the exhaust gas out. The next time the intake valve opens, there is less room for fresh fuel/air to enter and burn which means less power and causes a loss of torque. As the engine reaches higher RPM ranges, the pulses become less of a factor and that is why running open headers on a high RPM only engine is technically a moot point when discussing a street engine that will never see that type of constant high RPM.
In the case of the vehicle in this post and the fact it doesn't run a turbo, too large a pipe will cost you the absolute peak performance your set-up is capable of. I would refer you back to the link I posted from one of the largest exhaust component companies around that has researched the pipe size question for ages.