People seem to like to cite physics without actually demonstrating they are thinking about the geometry and physics of the situation. A few things of note in the context of powder (some of which is a bit of a repeat of some posts above):
1) Surface area good. Especially underfoot
2) Sidecut is bad. Or framed another way, the greater the proportion of total surface area underfoot the better.
3) A modern really fat waisted ski is not materially harder to turn in more than a few inches of powder than a narrow ski is on firmer snow. The physics of turning in soft snow as deep as your ski is wide is quite different than turning on edge on firmer snow.
4) Ski shape matters. My 5'7" spouse's Praxis Powders are, at their widest point, ten mm narrower than the La Nina is at its widest point. Despite the Protest's 128 mm waist. Stance width is driven by widest point. Not waist. And many - probably most - modern powder oriented skis have less of a waist to widest differential than other designs. So their widest point may not be materially wider (or even as wide) as a more conventional design - despite rather different waist widths.
5) "Edge" does not matter in powder. It matters for access (to or from) in the resort. Or, often, ridge tops and maybe valleys for things like heli.
6) With respect to surface area and powder - more surface, especially underfoot - is better. Except as constrained by practical concerns. There does get to be a point where a ski gets too long. Or where, out of the pow/slush/corn, edge and handling on firm snow matter.
7) For those citing stance width as a big deal. It might be worth considering that 1 cm is less than half an inch - so a ski that is 15mm "wider" (say going from 100 to 115) is 7.5 mm wider each side of the center. Or, rounding up, let's call it 1/3 of an inch. So the extra stance width you'd introduce by skis 15 mm wider at their widest (again, likely not the case with wider waisted modern skis) is a hair over a half inch total (between the two skis).
In short, 120mm underfoot and 175cm can make total sense - depending on intended use and skier size. And certainly makes plenty of sense for normal size women and smaller guys.
If anything, bigger skiers are cheated by the fact that other practical considerations limit ski designs so that we can not normally get as much proportional surface area as a smaller skier can.
IIRC Garywaynes are like 180 underfoot and 175 long. In their element, they are a hoot (not so much out of it).