The logic is that more surface radiating area gives deeper bass output. This is true to some degree though the implementation actually results in the DWM and main panels sharing lower bass output but handing off upper sub-bass output to the DWM which goes just a little lower than a MMG. This allowed Magnepan to directly sell and ship to customers (who were out of range of a dealer) a fully planar system that could go lower than the MMGs. Now that the 1.7i has a direct shipping path of ownership (and with some dealers willing to drop-ship other models), the super-MMG system seems to fill a smaller niche.
As for using a DWM with the larger panels, there is something to consider: the newer of the larger models have gone to all quasi-ribbon single-ended magnet panel setups. The DWMs, on the other hand, still use the push-pull magnets & wire panel setup. I lack experience with the full QR panels so ¯\_(ツ)_/¯ if I can say if that makes a meaningful difference. Given the DWMs crossover divides bass power between panels more so than bins power by frequency to each panel, more phase interaction issues are encountered in room setup potentially muddling any benefit to the PP magnet tech.
My cynical take on the whole DWM concept is that they still wanted to utilize their PP magnet panel machines and tooling when the majority of their speaker lineup is shifting towards full SE magnet QR panels.
Life is Lambertian