What limits the size of fish compared to marine mammals?

Question

The largest extant fish species is the whale shark (*Rhincodon typus*). With a length of 13 meters and a mass of about 20-30 metric tons, the whale shark is relatively small compared to the largest mammals, blue whales (*Balaenoptera musculus*, 180 tons) and fin whales (*Balaenoptera physalus*, 120 tons). Is there a physical, physiological, or anatomical explanation why fish never reached the size of marine mammals? Bigger isn't necessarily better, but I was wondering if there is a simple explanation why fish species are so much smaller than marine mammals.

Answer 1

I believe it could be partially due to skeletal structure but mostly due to ecological niches. There could have been larger fish in the past than what we see today, but nowadays large mammals such as whales fit a specific niche. So if a fish were to grow to that size, it would have a competitor that has already been there for millions of years.