Is that part of why there is so much push to breed small BCs and small shelties? Or is that something else? Just wanting to stay out of the 'BC classes'?
I think a big part of it is trying to get out of that darn 22" height class. Interestingly enough, what I'm seeing in our area is suddenly the "big people" moving into the 26" class. We've got a top competitor in our region who has a BC that I swear is half horse. He's not competing yet, but she specifically went out looking for a big dog. And she's not the only person I know. It makes me wonder if, in the next few years, the 16" and 26" classes will be the huge crazy ones. They've certainly gotten more competitive in the years I've been competing (not many, compared to a lot of people here).
As for the small dog/large dog thing - I think it is a toss up. I have friends who breed and compete Cavaliers, and do exceptionally well with them. They get a lot of flack for running small dogs and not getting "real" agility dogs. They run twice as fast and far as most big dog handlers, because you don't get the momentum and distance on the little dogs as easily. They train their contacts as hard or harder as anyone I know, because even little dogs can jump the yellow in big extension. Some things are easier with big dogs, some are easier with little dogs.