Bob Sapp had been training MMA, and therefore submission wrestling, for years by the time he fought Nog. Also Nogs game depends a lot on grabbing the wrists, which isn't very effective when you give up 100 pounds. Look at Royce vs Akebono. Akebono is a very accomplished sumo wrestle who had like 400 pounds on Royce, and Royce couldn't even close his guard. He still subbed him.
People that have never done any form of submission wrestling simply can't comprehend just how awful they are at it until they have tried it. Even hulking 300 lb monsters get made to look like chumps by 125 lb men because they simply don't know what they're doing.
With that said, wrestlers pick up submissions and submission defense very very quickly because they already understand general grappling movements and have developed all of the right muscle groups. So if Karelin just decide one day he felt like grappling with Werdum yes he would take him down and then get murder stomped, but if he took a year or so to learn the BJJ/catch wrestling/sambo/judo/whatever basics then yes he might be able to avoid Werdums subs on the ground. Even then, that's far from a guarantee.
