Elite boxers make far more than elite MMA fighters. There are at least eight boxers who make mid- to high seven figures or more every time they step between the ropes: Mayweather, Pacquiao, Klitschko (now retired), Joshua, Cotto, Ward, Canelo, GGG. It won't be long before Crawford is in that region, either.
The biggest difference in purses in boxing and MMA is that boxing has almost no middle class. Superstars make seven figures or higher per bout, and nearly everyone else is at five figures or lower. There are a few guys who are in the six-figure range per bout, such as Crawford, Chocolatito, Thurman, Danny Garcia, Mikey Garcia. But the vast majority of boxers make five figures or less per bout because the sport's pay structure is so top-heavy.
MMA has more of a middle class of fighters because the top-tier guys don't make nearly as much as elite boxers.