It took me a while to jump on the fat-skis bandwagon. I was perfectly comfortable on my 90-mm-waist sticks until I used them on a massive powder day. I couldn't keep up with the pack and my quads were killing me.
After that, I took a giant leap and went for the Line Pandoras. I was a little nervous about riding something so fat (115-mm underfoot) but after a few laps I realized how much more stable and reliable these are in crud and powder. Not to mention faster.
An Aspen core and thinner tip decrease the skis' weight making them easy for me to handle in deep pow. The 15.8-m sidecut give the skis a smaller turning radius, meaning I can survive trees and bumps by making sharp, tight turns.
In short, don't get hung up on waist size. If you want a fast, stable, and quick turning ski that works great in powder, the Pandora will get the job done.