I get that water helps support the whales giant weight but why did no fish become so large? Is it just a coincidence or is there something about being a mammal that helps it be large?
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There have been fish almost as large as blue whales, they’re just not around today. When we look at existing creatures we are just taking a snapshot of a tiny slice in time. It just so happens that certain marine mammals are the largest animals around today.
Any of the [Megaladon shark species](https://en.wikipedia.org/wiki/Megalodon) are thought to have individual examples which grew to over 20 metres length, which is equal to the average length of blue whales, though there are examples of individual blue whales reaching 30 metres long.
Exactly the same can be said for the extinct fish [Leedsichthys](https://en.wikipedia.org/wiki/Leedsichthys) which existed and then died out in oceans well over a hundred million years before any Megalodon showed up. Leedsichthys would have been alive during the middle part of the dinosaurs reign on land; it was contemporary to the earlier Stegosaurians for example.
Your original question could be reframed as “why have such large cetaceans only just started to be a thing at the moment?”, after all, we have no record of marine mammals (or fish) ever reaching such sizes at any point in the past as the blue whale does today. So blue whales aren’t just the largest animals currently around, they are likely the largest animals that ever existed. (Note: this only holds when measuring by mass, not by length. The largest sauropod dinosaurs such as Argentinosaurus were almost certainly several metres longer on average, though nowhere near as weighty because they had to support 100% of their own weight, unlike anything living in the water).
A few of reasons I can think of for certain baleen whales getting really large in very recent times are:
(1) the current climate and dynamics of the oceans producing huge seasonal plankton blooms which can be efficiently harvested by baleen filter feeders
(2) the switch to an ‘ice-house’ Earth system (ie. one with polar ice caps) results in large regions of very cold waters. Gaseous oxygen more readily dissolves in colder water, so animals can absorb more O₂ and grow larger without expending too much extra energy
(3) the extinction of certain apex predators when the climate system changed to ice-house mode, the number one suspect here being… you guessed it — Megaladon sharks. If I remember correctly, the evolution of certain orcas to reach their current average sizes is thought to be a result of Megaladons extinction. As with most things though, it’s pretty much guaranteed to be [more complicated than that.](https://www.google.co.uk/amp/s/amp.theguardian.com/environment/2016/may/31/secrets-of-killer-whale-evolution-revealed-by-genetic-study-orcas)
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