I've been wondering about this. I don't claim to be an authority on this...just an opinion about bike size. Generally I've been thinking that a DH bike should be sized more precisely to the rider because the rider should be pedaling more while descending, and stability is every bit as important as flickability to maintain speed. Now, for a FR bike the flickability is more important, so a smaller, more nimble setup is the priority. From listening to and seeing some of the FR stuff done by guys on this forum, it seems to involve more jumping and dropping where the smaller setup is an advantage for manueverability. True DH seems more about speed down and over terrain and involves a decent amount of pedaling at times to maintain momentum...therefore a bike more exacting to the rider's size. And while DH requires manuevering and decent flickability, it is equally weighed with stability for maintaining speed.
Maybe some of this comes down to how one defines DH and FR riding, but to me DH is closer to racing...speed...and FR is more about flickability...stunts and moves.