My sense is that kbouressa's spot on; instructors like newbie kids on really short skis so they can get them from pizzas to french fries as soon as possible. So not just about size and leg coordination, but learning curves.
My 6 year old, 44 inches, had been on 90's from age 4 until this past week. Did just fine even though they only came up to his sternum. Now he's an early 5 on 107's. And the ski school just started my 2.8 year old on 70 cm skis. At that age they just get them used to gliding forward while they walk alongside, shuffling on the flat, tie the tips together to encourage pizzas. Only do it for a half hour to 40 minutes, then time for snow play.
Which at that age, IMO is all it should be about. The idea is to encourage love of our sport and its world, not make them into racers as soon as possible.