There are also some other reasons why a designer might use multiple smaller woofers rather than a single large one. If the woofers are to be front-mounted, the physically narrower drivers allow for a narrower baffle, which in turn permits theoretically wider dispersion and less diffraction from mid and high frequency drivers mounted on that same baffle. Smaller cones of lower mass but greater stiffness than larger cones allow easier motor control over acceleration/deacceleration. The smaller radiating diameters of these 'mini-woofers' allow them to be carried up higher in frequency before the crossover rolls them off without paying as much of a penalty in reduced dispersion (frequency beaming) as would be the case with a larger-diameter woofer - thereby letting the midrange driver roll in later with the attendent power-handling benefits of its not having to cope with some lower-down frequencies, yet still maintaining smooth power (polar) response through the lower midrange. And using multiple woofers of a single size and simply varying the number of such drivers employed in different models up and down a manufacturer's line increases production efficiency over needing to have many different driver sizes.
In all cases, whether with multiple or single woofers, the possible LF performance limits will have a lot to do with the total driven cone area, the total motor power applied to that driven cone area, and the total enclosed cabinet volume (as well as the particular cabinet design and tuning, and the performance attributes of the drivers used). So for your question, the larger speaker models with more of the identical woofers exhibit increases in all three of these parameters, and therefore with all other things being equal should be able to go deeper, louder, and with less distortion than the smaller models with fewer of the woofers.
(The same sorts of things hold true for midranges and tweeters too, even though the choice there is not between multiple smaller drivers and single larger ones, and excepting that of course LF extension and cabinet volume are not part of the critical equation as with the woofers, but multiple mids or tweets can afford higher headroom and lower distortion just the same. But with their relatively small diaphragm excursions, it is not generally necessary to increase these capacities in home speakers beyond that achievable with single mid/HF drivers, and most models featuring multiple mid/HF drivers do so more for reasons of shaping radiation patterns, such as in a D'Appolito mid/tweeter/mid array or a THX tweeter line.)
But as to whether multiple smaller woofers are somehow "better" than single larger woofers, I don't think you can make such a generalization. For one thing, there is no getting around the necessity for greater cabinet volume if really want to go low with authority (barring the use of compensatory active EQ as in ultra-compact subs, which increases distortion and lowers headroom), and the most efficient way to package this internal volume is not to restrict baffle width if you don't want to dictate a speaker that's very tall and deep. By the same token, one 12" woofer on that wider baffle makes a more space-efficient package than do the four 6" woofers that would be needed to total the equivalent cone area. And for what it's worth - and for reasons that I'm not sure I fully understand (maybe others can elaborate) - all the speakers that I've ever heard (or are reputed) to display truly whomping deep bass unfailingly have physically large woofers (8"-10" min., more likely 11"-15", whether singly or multiple), not a larger number of smaller ones.
In all cases, whether with multiple or single woofers, the possible LF performance limits will have a lot to do with the total driven cone area, the total motor power applied to that driven cone area, and the total enclosed cabinet volume (as well as the particular cabinet design and tuning, and the performance attributes of the drivers used). So for your question, the larger speaker models with more of the identical woofers exhibit increases in all three of these parameters, and therefore with all other things being equal should be able to go deeper, louder, and with less distortion than the smaller models with fewer of the woofers.
(The same sorts of things hold true for midranges and tweeters too, even though the choice there is not between multiple smaller drivers and single larger ones, and excepting that of course LF extension and cabinet volume are not part of the critical equation as with the woofers, but multiple mids or tweets can afford higher headroom and lower distortion just the same. But with their relatively small diaphragm excursions, it is not generally necessary to increase these capacities in home speakers beyond that achievable with single mid/HF drivers, and most models featuring multiple mid/HF drivers do so more for reasons of shaping radiation patterns, such as in a D'Appolito mid/tweeter/mid array or a THX tweeter line.)
But as to whether multiple smaller woofers are somehow "better" than single larger woofers, I don't think you can make such a generalization. For one thing, there is no getting around the necessity for greater cabinet volume if really want to go low with authority (barring the use of compensatory active EQ as in ultra-compact subs, which increases distortion and lowers headroom), and the most efficient way to package this internal volume is not to restrict baffle width if you don't want to dictate a speaker that's very tall and deep. By the same token, one 12" woofer on that wider baffle makes a more space-efficient package than do the four 6" woofers that would be needed to total the equivalent cone area. And for what it's worth - and for reasons that I'm not sure I fully understand (maybe others can elaborate) - all the speakers that I've ever heard (or are reputed) to display truly whomping deep bass unfailingly have physically large woofers (8"-10" min., more likely 11"-15", whether singly or multiple), not a larger number of smaller ones.