I got to see an Ocean Sunfish (Mola mola) up close and personal at the Monterey Bay Aquarium recently and I fell in love. These things are the superlative of superlatives. They are the biggest bony fish, they lay the most eggs of any vertebrate, and grow the most from egg to full size of any vertebrate. Yes, they're silly looking, but the whole thing about evolution is, whether we can figure out why or not, things don't survive if they aren't good at /something/.
Sea turtles and mola mola, despite having deeply different lineages, actually share a lot of characteristics. They have the same shaped flippers, they both have beaks, they both rely on their size to protect them, and they both create lots of babies in the hopes that only a few will survive (though, Mola mola take this to a different level.) They're also both basically the same shape, which is super weird.
And here's a wild thing, even though sea turtles evolved from land-dwelling reptiles, mola mola-like fish have actually been around for 100 million years while sea turtles have been around for over 200 million years.
Now, I'm just guessing here, but this is possibly because they share a food source. That food source is low quality but abundant. It also doesn't move fast. So the mola mola evolved not for speed, but for efficiency. These flippers (like the ones that turtles and mola have) don't move you fast, but they also don't use much energy.
The fecundity of sunfish might also have something to do with the variability of their prey. Jellies are notorious for being extremely abundant year and then disappearing the next. In a situation like that, you want to have the ability to rapidly increase your population and rapidly reach the age of maturity so that you can take advantage of prey population explosions.
Research Assistance from Deboki Chakravarti