Why would "hip high" be a bad thing in a young horse?

Another question's answer made we question this. I know that a mature horse that is hip (or ) high will have trouble collecting, and if extreme, could lead to lameness issues BUT, I consider "hip high" to be a good thing in a youngster (the horse in the question that inspired this question is 18 months) who you expect to continue growing. In fact, I usually consider it a clue that the horse still has some growing left to do. Where is my thinking off on this?