I think that if a boot is a couple shell size to big...which I see everyday....it is then 100 percent a boot issue. To anyone who think differently go put on some boots that are to big and try to ski.
with that said tons of people ski on properly fitted boots and still ski back seat.
Definitely a rental issue and since comfort is not fully understood that gets in the way of a more beneficial fit. People in big soft boots just have way too much range of motion and little control over any of it. We tell them to move forward , they rock forwards with their shoulders and the backside heads the other way flexing all the way.
Man, that sounds kinda Christmas -ish. Anyway sloppy boots suck big time but it's what they serve .