Here we go again.
Back pressure is never, ever a good thing in an exhaust system.
But adding bigger pipes will not necessarily reduce backpressure. Wave propagation, timing, cam events and several other factors all effect this.
Here's an interesting report from CAFE and the FAA on aircraft exhaust system tuning. The "Background" section specifically addresses backpressure as always being a negative thing. (they even mention bikes)
http://www.cafefoundation.org/aprs/epg.pdf
Ideally, what you really want is below ambient pressure (sometimes called "suction", but there is no such thing, only a lack of pressure) in the exhaust header when the exhaust valve opens. This will help better scavenge the combustion chamber and depending on your overlap in the cam, might even help pull the intake charge down.
Bigger pipes work better with higher exhaust flows. At low flows the pipes don't have a high enough exhaust energy to keep the gas moving down the (big) tube, and backpressure results at low RPM, hurting efficiency, tq and ultimately HP.