the only things that a wider ski limits is the ability to transition from edge to edge quickly, and the ability to drive through crud. The wider the ski the slower the transition. Also, with a wider ski, there is more surface area trying to drive through crud, often times making it want to float over the crud (instead of drive through it). Any other aspect of the ski's ability can pretty much be made identical (radius, flex, tortional rigidity, etc).
Mid-fats have earned a good reputation because they are fat enough in the waist to get a decent float, but narrow enough to get a pretty quick transition, and yet still be able to drive through the crud, making them a very useful ski.
Typically I think of 63 (I think is the narrowist I've seen) to 68 as narrow, typically race oriented skis, 70-82/85 is a mid-fat, more all mountain, all purpose skis, 85+ more powder/softsnow oriented.
you gotta realize the waist measurements that I just mentioned though are more of personal opinion than design numbers, hope it helped.