"Necessarily" is a standard that's too strict and binary.

The relevant question is to what extent brain size facilitates and is correlated with intelligence.

It doesn't seem to be irrelevant. Large brains are correlated with higher IQ within humans. Also, humans' intelligence came about with an almost doubling in brain volume over two million years.

Across very different species there is a weaker correlation but it still seems to be positive. Whales and dolphins are far smarter than ants. Octopuses seem to be an outlier in that respect.

I think it's fair to say that the probability of an "Einstein" is positively correlated with body mass, and therefore negatively correlated with population density, at least up to a point (whether that correlation turns negative beyond a point can be debated).

Bird brains, such as parrot and crows so that brain size is not everything. A crows brain is smaller than a chimps in overall size and body:brain ratio, yet crows show more complex social behavior, problem solving and tool use (despite not having hands). Bird's brains are much denser than mammals, so it's less size and more number of neurons, and the complexity of how these neurons are networked.

Outliers do not invalidate a correlation.

This reminds me of Douglas Adams writing about how "on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much--the wheel, New York, wars and so on--whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man--for precisely the same reasons."

Maybe dolphins/whales and elephants are in fact smarter than us but are stuck underwater with no fire and no tech tree or on land without opposable thumbs

I feel like we’d be communicating with them by now if that were the case. The dolphin-socrates would have a YouTube channel campaigning for dolphin rights or something, via a human translator and camera crew.

Our world would look like that Netflix movie “Bright”, with multiple intelligent species living and working together. (The movie was bad by the way)

Motivation could be an issue. What if they do not want to communicate with us? What if we are too slow for them? Dolphins produce burst pulses that have interclick interval of just few milliseconds. We cannot even hear these individual clicks and for us the burst pulse sounds like a creaky door. But they, supposedly, can hear individual clicks. We do not understand or use echolocation, they do, so their "dolphin-socrates" probably would have their own allegory of the cave that is not based on what individuals can see.

Humans have the computers and machine learning experience. Why aren't they making software to figure it out? And dolphin/machine interfaces? Touch screens for elephants?

While that would be awesome, these foreign intelligences might have too little in common with our wiring to communicate in any way.

Also, we’ve been shaped by our tools since day one, and these animals use little or no tools.

We already talk to gorillas via sign language.

We’re pretty close to gorillas.

We’re very different animals from dolphins and elephants.

We may have nothing at all in common with extraterrestrial beings.

An overlap of concepts is needed for two entities to communicate.