A little more from explanation from 2012 discussion I posted in:
For simplicity lets assume that pressure to the ski is transferred thru a platform, which we call the boot. This platform is about 70mm wide. In order to be efficient thru a turn in terms of body/joints/edge angles alignment, the whole system (skier-boot-bindings-ski) should be aligned to excerpt pressure perpendicular to the ski while ski is on the edge. Again, for simplicity, lets assume that as long as the effective edge of a ski is under the boot (not necessarily under the boor center) with the pressure being applied perpendicular to the ski surface - the system is in alignment:
Now, what happens if ski gets wider? Since the effective edge position is outside of the boot platform, there is a torque, which rotates the knee outside of a turn around the edge. This rotation requires skier to compensate for it with additional force applied to the knee towards inside of the turn:
Running thru the math, with multiple assumptions to make of coarse, this compensation force could be translated into additional 2 to 10 lbs applied to each knee for each turn for each additional 20mm of a ski width (depends of the turn shape, skiers weight, speed etc). Note that this force is applied LATERALLY to the knee! Not a best exercise for the joint ;-)
When show is softer and ski can dig dipper into the snow, we are getting the effective edge back under the boot and the torque gets minimized:
So, too fat or not too fat - it all depends on how deep the ski digs into the snow, which of coarse depends on snow density and skier's ability to excerpt the pressure on a ski AND how much of knee beating the skier can and/or willing to take ;-))
Even though this somewhat very simplified description is applicable mostly for a CARVED turn, some of it takes place for skidded turn as well - not with such force, but for a longer periods of a time.
Now, the opposite - when does a ski become too narrow?
Once the snow is soft and deep enough to allow ski to submerge into the snow, pure caved turn becomes almost impossible and turning mechanism changes completely - using gravity skier "falls" thru the snow at the apex of the turn, then for the transition skier compresses to push the snow down to create a platform from which he/she "jumps" into the next turn using momentum (don't take this as a literal jump). When the combination of terrain slope, snow density and viscosity, skier's speed and strength becomes such that ski width allows ski to sink "too" deep into the snow during the compression phase that there is not enough momentum (or strength) to get out of the snow into the next turn - this is when ski becomes too narrow for that particular pitch, show type and that particular skier.
With that in mind - it all depends of what is TYPICAL snow condition someone skis, TYPICAL pitch someone skis and what sacrifices one is willing to make when a NON-TYPICAL conditions is encountered.