Thanks. Great question, I often wonder the same thing as well. I can say with complete honesty that I have no idea.
Safety is a concern. I suspect I wouldn't want to be on a DW run on a smaller board without boat support. Windsurfers have broken gear and had to spend the night on the SF bay.
For surfing heck yeah, smaller boards seem like the call. My shaper showed me a tiny board he is making for prone foiling...