Getting fat....or notI think we have to start with the premise that every ski dimension (length, width, sidecut, stiffness, torsional rigidity, etc.) is a compromise. Back before I switched to tele, I had a pair of 205cm Salomon 9000 GS skis. They would carve GS turns, but anything shorter would be skidded, not carved. They were stiff and narrow, optimized for wide radius turns at speed, good for groomed cruisers and OK for slashing through crudded up powder, but not so good for other conditions, especially moguls. Nevertheless, they were fun.
Now, even though I live back east, most of my skiing days are in Utah, and my current skis are about 180 cm by 130-98-120mm. They are fairly stiff, but with adequate width to float wonderfully in deep powder. The stiffness lets them blast through crud, and when I want, I can crank a carved p-turn or two when I get tired of dropping a knee on the run-outs. Again, however, these skis are a compromise. They are really not ideal for tight, icy moguls or the tight trees of MRG. If I spent a lot of time doing that type of skiing, I would consider a narrow-waisted, ski in the 175 cm range. Then there are snow-blades, which take the short-radius twitchy ski to an extreme.
At the other extreme are the backcountry skiers who prefer skis optimized to bottomless powder and who have the leg strength of a solid athlete. I have a friend who has skis that have a 130mm waist, skis 90% backcountry, and is very strong. He can handle them in-bounds, but that is not their strong point. Even further out the curve are skis like Spatulas, which have the inverted camber of a waterski, and pretty much the same width. These are pretty much useless on anything other than deep powder.
Then, there are the designers who have developed asymmetrical skis. The most extreme are the ScottyBobs, where the inside edge of each ski is a good 15" longer than the outside edge. G3 also makes an asymmetrical ski. I have tried the G3 reverend, and found it to be a surprisingly nimble performer.
I won't even go into the issue of weight and durability, foam-core vs. wood core. This may make a big difference for a backcountry touring ski.
All of these designs are attempts to optimize for one set of performance characteristics or another. It is hard to say which is better in the abstract. Everyone has to decide what they want to do with their skis, and buy accordingly.