I don't think the W22 cabinet needs to be any bigger, although I'm not arguing that from a physics standpoint... It's just that it's a pretty big hunk of wood hanging out from the wall already. The speaker also has 10.5" of woofer, although not having done high school math in over a decade I can't tell you that would equate to in a single driver (about 7" I would guess). Compare that to your typical bookshelf or in wall speaker, or even satellite.
I don't think anybody has ever stated they go as deep as the standard model, but I disagree they do not go low enough to integrate properly with a sub, especially for music. With movies they don't hit you in the chest like a big floorstander would, but I don't believe the M22's do either.
Edit: Picture whoring for effect
