I remember one of my old TKD instructors trying to get me to spar like him. I'm significantly taller and heavier and it just didn't seem natural to spar the way he did.
I think that you have to sometimes accept that you will adapt to your body type.
That said, are smaller people naturally faster? I don't think so, I think that you work on speed and timing no matter your size.
I wish there were some readily available videos of TKD legend Tony Sewell in his prime. Below is what I could find from youtube, but this is 15 years after he retired from competition.
http://www.youtube.com/watch?v=XVFhB8pCs8o
He was a definite heavyweight, but could do the most amazing kicks such as high section jump spin back kick and before he landed rechamber and do a second kick before landing. He could do it off either leg. I think there is a section on Clash of the Titans 10 where he does an amazing kicking demonstration with Pele Reid.
Pete, was the heavyweight kickboxer my pal Norbu perchance? He's lighter and faster these days which keeps me on my toes every sunday!