You have to raise and kill more chickens per pound of meat, but aren't they much, much less intelligent than cows?

They are, but while intelligence does likely have a correlation with capacity for suffering, it isn't the same thing. For example, a human infant is much less intelligent than an adult, but we do not typically believe their capacity for suffering is lower than an adult.

Cross-species capacity for suffering is still under heavy research, but the general consensus is that birds (including chickens) have a comparable capacity of suffering to mammals. This is considered likely true or at least plausibly true for fish as well, though their brains are sufficiently different from mammals and birds that there's still more research going on to understand it more. Animals that likely have no capacity for suffering are bivalves, for example.

It'll take a lot more research to figure out the concrete answer for those in between (e.g. insects, where the latest I've heard is that the most they can suffer is similar to an annoyance or mild pain from a human perspective).

I suppose I care less about the suffering of less intelligent animals than I do about more intelligent ones. All else being equal, I’d like chickens to be raised better, to the point where I pay a lot extra for it (though this is partly pure selfishness, I think chickens who’ve gotten a lot of exercise taste much better), and I think that sadism against chickens is still sadistic and evil, but even if they can suffer, I’m not going to put them on the same level as eg a suffering whale.

And babies past a few months of age are pretty remarkably intelligent, albeit still a bit uncalibrated.

It is plausible chickens have less moral weight than whales. But it's not exactly because of intelligence -- it's their capacity to suffer. I know intelligence is very tempting to use because it's a very good proxy in the extreme cases (e.g. a sponge vs. a human), so it's usually our first intuition.

But if you probe into it, I believe you'll find it's an artificial distinction that rather quickly falls apart. If your moral values ascribe negative value to the suffering of, say, both a human and a dog, then the only consistent viewpoint is to ascribe negative value to any suffering of a materially similar quality -- i.e. what matters on a moral level is the capacity of suffering for the species or individual.

There's a few uncomfortable thought experiments you can think through on this topic. E.g. you can imagine genetically engineering a human as dumb as a chicken, or perhaps a traumatic brain injury causing that, but with their mental capacity for suffering just as intact as yours or mine. Would their suffering count for less? Why would it?

As for chicken vs. whale, it may be the case that a chicken suffers less than a whale. More primitive intelligence may imply more primitive suffering. But what if it doesn't for this case? How confident can you be it does imply that, given both how chickens react to pain and stress and the neurological research that's been done on them? And even if it less, by how much? A factor of two, ten, forty, a thousand?

Those questions may seem too philosophical, but they can't be ignored in the face of a recommendation to eat 40 extremely-poorly-raised chickens per year instead of 1 reasonably-well-raised cow. And given all we know of animal cognition, it's reasonably to assume chickens have a lower capacity to suffer (and also to assume it's the same -- there's still uncertainty), but not so much lower that that calculus is anywhere close to being worthwhile.

Yes, but most meat chickens and egg chickens live out their lives in cages, without enough space to turn around with their beaks mutilated. Most cattle live the enormous majority of their lives in a kind of cattle paradise in comparison, with ever present adequate food, no predators and some modicum of medical care.