I've been thinking about the guys riding bigger doms in bigger waves. I reckon it's to do with the length to width ratio on bigger sizes.
For me, on a 5'8" x 20" at 6'0 tall it's a short and WIDE board for me to control at speed on solid waves. But for someone 6'0 and 90+kg on a 6'4" x 21" you've got all that extra rail length and only gained an inch width. So if the volume to weight ratio for the board to the rider then clearly the bigger guy on the longer board is going to have much more success in larger waves than the guy on the super short and comparatively wider board.