what receiver? what your sub does during 2-channel playback can vary slightly from receiver manufacturer to manufacturer, and is most likely dependent upon another setting or two, besides speaker size.
essentially, setting a speaker to SMALL in the receiver will redirect any sonic information for that channel that is below the receiver's set crossover to the subwoofer. if you set a speaker to LARGE, then none of the sonic information for that channel is redirected and the speaker is sent a...