http://www.sciencemag.org/news/2018/05/ancient-fish-was-bigger-whale-shark-and-faster-scientists-ever-imagined

Scientists have long struggled to explain why bony fishes are so small: The heaviest—the ocean sunfish—is just 2.3 metric tons, but cartilaginous fishes like whale sharks can weigh up to 34 metric tons. Now, a new study of an ancient giant suggests this modern difference is merely an evolutionary accident.

Bony fish, which make up some 95% of all fish species, might be constrained by their metabolism according to one argument. Larger animals generally have to make do with less oxygen per gram of tissue; because bony fish seem to have higher metabolic requirements than sharks, it might simply be impossible for them to grow much larger than the ocean sunfish.