Brain size doesn’t correlate at all with intelligence. I say this as someone with ridiculously large brain, because I have a large head (outside 3XL hat sizes) and am already a large person (6’2", wide shoulders), so it’s like “I wish…”. Further, head size and body size tend to correlate (and brain size and head size are pretty much 1:1), and black and white people are the biggest, generally, so would have the biggest brains, so it would never have been a great line for racists. Hell, Neanderthals, who were probably not stupid, but certainly not the same sort of thinkers as modern humans had much bigger brains than modern humans.
I think sexism would have been the issue rather than racism. Because women do tend to be a fair bit smaller than men, on average. But again, there’s no correlation, so…
Research of dubiously ethical value does get published pretty often. A few years back there was a study where it was claimed people with wide faces, especially men with wide faces, were basically pathological liars and violent. Turned out it was a bunch of overhyped nonsense which was improperly conducted, but that’s often the case with dubiously ethical stuff, because it usually starts out of a desire to prove a particular point (even if an unstated one), and that’s like building on sand.
One thing worth remembering is “extraordinary claims need extraordinary evidence”, which isn’t so much a literal thing, as, if you’re going to say your research indicates about 30% of the population are monsters, you better have some damn good evidence. And they never do.