Macrojack: What you're essentially saying is that if you turn down the volume, the cabinet will move less. D'oh! :-)
Seriously though, if you want to achieve the same lower-frequency SPL in-room, you have to move the same amount of air. Can be done through either larger driver area or greater driver excursion. Both have engineering challenges if you want to reduce distortions of the driver (and therefore of the reproduced signal) in several forms. So some designs involve using many LF drivers, which can reduce both area and excursion on a per-driver basis, at the expense of increased cost for the multiple drive units and larger cabinets to house them all (which, to the topic at hand, obviously makes for heavier speakers). But the other trend in the industry has been toward EQ'ed subwoofers with small, inconspicuous cabinets, and these drivers have some amazing levels of peak excursion, at the expense of accepting increased distortion in an area where the ear is less sensitive to it, and targeted primarily toward end users who care more about movie soundtrack FX than music quality. Some of these little monsters can move around the floor under high SPLs if not spiked.
Seriously though, if you want to achieve the same lower-frequency SPL in-room, you have to move the same amount of air. Can be done through either larger driver area or greater driver excursion. Both have engineering challenges if you want to reduce distortions of the driver (and therefore of the reproduced signal) in several forms. So some designs involve using many LF drivers, which can reduce both area and excursion on a per-driver basis, at the expense of increased cost for the multiple drive units and larger cabinets to house them all (which, to the topic at hand, obviously makes for heavier speakers). But the other trend in the industry has been toward EQ'ed subwoofers with small, inconspicuous cabinets, and these drivers have some amazing levels of peak excursion, at the expense of accepting increased distortion in an area where the ear is less sensitive to it, and targeted primarily toward end users who care more about movie soundtrack FX than music quality. Some of these little monsters can move around the floor under high SPLs if not spiked.