Jake Paul really is not nearly as big as you're making him to be. Most of his fights have been well under 200 pounds, and quite frankly, when he's fought bigger guys he looks pretty fat for a boxer. He's a very modest sized middleweight in MMA.
He's also a mediocre boxer by professional boxing standards, while Topuria is like...probably going to be a top 5-10 MMA fighter of all time? Topuria is going to be knocking guys out who are natural 180+ pounders (who are some of the best fighters in MMA). The only career boxers Paul fights are legitimately club level boxers, and he lost to one of them.
I dont think Jake Paul has a particularly great chance in a fight with no rules.
There have been guys around Topuria's size who are much worse who have beaten fighters significantly bigger than Jake Paul in MMA fights (see old fights in Dream, Pride, Pancrase, UFC etc). Topuria isn't Demetrius Johnson small.