Typically setting the speaker size to large removes a filter to allow a full range signal to go to the affected speakers. For instance I have Klipsch RF-82 speakers with a response down to 37hz. If I set them to large in my system then they receive full range input. If I set them to small, I have to tell my system where to limit the signal to them. What I typically do is set them to small and cross them over at 40 or 60 HZ. The advantage to that is the below the crossover frequency the LFE signal is sent to the subwoofer which has its own amplifier designed to power those power hungry low frequencies. That saves a lot of power in my amp to supply to the mains which means I can play my system to its full potential without detriment to the sound or challenging the amp or speakers to play outside their designed areas.
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Music system
Model 990/7500/Magnepan 1.6 QRs/Technics SL1200 MK2/Aperion S-12 Subwoofer/OWA3/Sony NS75H DVD
APC H15 Power Conditioner
TV System
Large Advent Loudspeakers/ Polk center/Monoprice surrounds/Panasonic Viera 42 inch/Onkyo HT-RC260/Sony BDP S590/Directv
Home Theater System
Onkyo PR-SC886/Outlaw 7125 Klipsch RF-82 L/R,RC-62 center, RB-35 SR/SL, BENQ HT1075, Outlaw LFM1-EX/OPPO BDP-83/Directv
Harmony ONE
Blue Jeans and Monoprice interconnects
APC H15 Power Conditioner