I understand, generally, *how* they got that big but not *why*. What was the evolutionary advantage to their massive size? Is there one? Or are they just big for the sake of being big?
In: Biology
Counterintuitively, a large part of it is that their food is so small.
It takes a *lot* of plankton to meet metabolic needs, so a larger collection area (eg. mouth with baleen) is useful, but this means a larger body size, which means a greater food requirement, which means a larger food collection system, and repeat until you run up agains the limits of biology.
There might be a sexual selection component to it. If female blue whales prefer the biggest mates, this starts a virtuous cycle where their offspring will have the genes for “liking the biggest males” and “being big”.
It’s the same idea behind peacock tails. Female peacocks started liking males with the biggest tail feathers, so they reproduced and the offspring carried both of those genes, amplifying the effect.
You have to remember that the only advantage evolution cares about is reproduction. Sure, if you are harder to kill, you have more time to reproduce, but let’s take out the middleman. I will say it is speculation on my part that this is the same for blue whales, but the answer might just be “Fashion!”
The ELI5 is: there was once a whale born abnormally big and it was able to get more food than other whales. It had babies who were also large and also could get more food than the others. Since they all compete for the same food source, the bigger whales survived and had more babies and the smaller whales died out.
Repeat this over and over with them getting bigger and bigger over millions of years and you get the modern blue whale 🐋
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