certainly more than 85% of people showing at my local mountain complex (Aspen) are in boots that are at least one size too big and in a large number of case mores. How does this happen
1) skiers (including the vast majority who paste here) have a very limited understanding of body mechanics which all depend on a transparent ability to maintain and control edge. Knowing next to nothing regarding technique (and much of what they think they know wrong) they turn to
2) some ski store "kid" proclaiming to be an expert proclaims to a nodding customer that comfort is the crucial attribute. Nothing is so comfortable as a boot two sizes too big on a ski shop floor. But that same boot ABSOLUTELY limits the aspiring skier to low intermediate at best.
3) so prevalent is this that ski school instructors (also largely limited - I mean 30 years of swishing your rear end as you skid turns on groomed trails) stress skidding and checking ... and if feet are somewhat parallel - accolades and pictures follow. People pay perhaps $700 for a full day private (the cost of a fairly good boot and orthotic - if bought on sale) with no chance whatsoever to do anything other than push their skis out in front of them and as they steer across the fall line and then edge set (like a hockey stop) for the brief second they are over the edges before the skid to the other side.)
And while on my somewhat snooty critique of posts on this site, even posts on which ski is the best ski avoids the key consideration -- SKIING IS ALL ABOUT REBOUND. To determine a good ski set it up (shovel and tail) on blocks (say like two bricks turned sideways. then stand with boots at camber. Push up and down - like a kid jumping on a trampoline - but not leaving the ground. If the skis fall to the floor they are too soft and will not hold up (skid and chatter). If they do not bend, they are to stiff - you will be unable to set and edge and then LOAD (set and angle the edge). Sorta like a diver pushing down on a diving board. Augment this with how aggressive you are and how much you weight. In general the common advise is to choose a ski which reaches up to your eyebrows. Poor advice - chose a somewhat longer ski - give up ease of steering for ability to maintain a smooth running edge.
Oh well --