Once a fight goes to the ground, as it often does in MMA, it's much more technical in nature and someone who has put in a few years of effort into grappling techniques can trounce larger opponents by doing things that they simply aren't prepared to understand. The muscle memory and observation to understand what the situation is and how to move through it takes persistence over time to develop.

So my guess for this matchup is strongly weighted towards a more prepared Zuck being patient, finding openings for a takedown and then winning by submission. That said, it isn't a jiujitsu match, and there won't be gis, which shifts around the techniques available. Elon has a chance if his claim of having studied some martial arts as a teen is true, but he is also a big liar and equally likely to cancel at the last minute.