Here's my 2 cents.
Surfski paddling basically has it's origins in SAF and Australia, or at least traditionally that has been the case. The overwhelming number of top-level ski racers come from these two countries. I chalk this up to two reasons. Proximity to open ocean w/big, long period swell, and a longstanding tradition of lifeguard spec racing, as an entry to ocean racing. Not to mention Saffo's and Aussie's typically have pretty gung-ho mentalities about adventure sport and are very comfortable in the ocean. Now ask yourself, what sort of genetic ancestry does the typical (white) South African or Aussie have. They are pretty much descended from northern European stock. Have a look at the dutch, danes, swedes, germans, and many brits. Tall, long torso, broad shoulders. I don't believe it is necessarily the case that larger people make faster ski paddlers, but rather that, coincidentally, many folks from the two places where ski is most popular happen to be large framed. And as a consequence of sheer numbers many of these people arrive at the top of the heap, (and also design boats largely for people like themselves.)